Boot the Bourgeoisie
You can't just ask nicely for the bourgeoisie to give up their power over you.
As such, you can't just vote in communism. If this were viable, then it would mean the bourgeoisie would have nothing to lose by resisting as much as they could. They would direct as much wealth as possible towards the maintenance of capitalist control.
Would it be effective? Well, yes, it has been.
Devoid of bourgeois interference, the election cycle should gradually drift leftwards. The definition of the left is that it attempts to improve the lives of the largest groups possible, so its policies would generally be more popular. Rightist positions would simply be... proven wrong. Society would eventually find a solid balance between improvement of production, and the betterment of lives. Our social issues would gradually fall away, and everything would be great forever!
This has not happened!
Why not Strive for Electoral Victories?
The bourgeoisie has strong control over the election process and the governmental processes afterwards--they get, for the most part, to decide what happens in the government. What makes them allow a left-leaning candidate is that they think it would cost too much money to fight back, or that the left-leaning candidate's policies would reduce radical sentiment enough that it would be worth the reduced rate of profit.
As such, left-leaning political parties who want to take power are incentivized to find and promote whatever candidates will win the most popular support while doing the least to harm the long-term interests of the bourgeoisie. This means that, when e.g. the Democratic Party is smart about its decisions, concessions to the working class will only be given insofar as they prevent further leftward sliding.
People oftentimes look at social democracy projects--like those in Sweden and Norway, presently--and wonder why we can't just have that sort of humanitarian capitalism everywhere. But a humanitarian capitalism was only ever modeled to reduce those countries' workers' sympathies towards the USSR. The Scandinavian countries, and West Germany, were built up as social-democratic paradises, until the USSR fell: now they all have a growing problem with the far-right, as with the rest of Europe.
Always, the bourgeoisie will do everything in its power to prevent a positive feedback loop from forming between left-wing policy and left-wing attitudes. When a candidate is unable to find the balance where their policies would satisfy the populace, rather than invigorate them, they are tossed out. When a candidate's left-wing politics are not needed in the first place to pacify the population, they get tossed out. See how center-left politicians like Bernie Sanders and Jeremy Corbyn have been treated by their party apparatuses; see how, recently, progressive US House member Jamaal Bowman got primary'd by the opposing candidate, whose name was Fifteen Million Dollars.
Okay, you might say--so there isn't going to be a positive feedback loop, inherent to the governmental system, leading to socialism. But that doesn't mean an electoral transition is impossible. It just means it would take more class consciousness, and more willingness to make it happen. What is actually stopping an electoral Communist victory?
Bourgeois Influence on Shooting People with Guns
There is an important difference between a populace being willing to vote for socialism and being willing to fight for socialism. The power of the proletariat comes, ultimately, from being able to lower the profit margins of the bourgeoisie when they don't get their way. They can run strikes and boycotts, they come with a built-in army to resist oppression, they will only take so much before fighting back in various ways.
The bourgeoisie maintains democracy because it keeps the peace, and keeps the proletariat out of their business. If a populace is willing to vote for, but not fight for, socialism, then the bourgeoisie is incentivized to use all the tools at their disposal to ignore the will of the government. This usually manifests two ways: make sure the socialist party that was voted in does not actually accomplish anything, or to subvert democracy entirely.
They have substantial means to complete both of these ends.
Due to their inherent advantages throughout the electoral system, they can place forth candidates who appear socialist but whose policies will only ever provide compromises. A political party that calls itself socialist has the same incentive as any other electoral party to appease both the proletariat and the bourgeoisie: while such a party might have to hold itself to higher standards, it can still end up as a social-democratic project that provides workers with various improvements without ever taking the ultimate decision-making power away from the bourgeoisie. (And if it's a first-past-the-post voting system, you only need to be 1% better than the other team!)
Even a strongly committed socialist candidate, though, may find great difficulty in making functional changes. The differences between capitalist society and socialist society are deep and far-reaching. Even the Communists who took complete power have in the past taken decades to flesh out their governmental system. A government under a capitalist system lacks the wealth, resources, and infrastructure to rapidly implement a socialist system--and, the whole time, the capitalists will be fighting back.
Due to their inherent advantages throughout real life, the bourgeoisie can overthrow the government. If your party is attempting to overthrow the bourgeoisie, then it needs to be able to handle everything the bourgeoisie throws at it, including: funding of right-wing terrorism, funding of coups and separatist movements, bribery, strong editorializing of mass media, and more. If the interests of a foreign bourgeoisie are involved, then that group gets even more options: funding economy-debilitating strikes, imposing sanctions, sending in foreign armies. The 1973 overthrow of Salvador Allende in Chile shows the power the US bourgeoisie holds over its neo-colonies: The CIA worked with an AFL-CIO project to fund a trucking strike and an airline strike, crippling the economy and diminishing mass support for the Marxist president, and then they ran a traditional coup to install Pinochet as a fascist dictator.
It is thus that every attempt to institute a genuine socialism will end up in violent conflict with a bourgeoisie that is willing to dedicate all of its resources to remaining in power. This could mean staving off a coup, or it could mean fighting a long civil war. The success of the socialist movement, in this moment, depends on the willingness of the proletariat to fight.
The willingness of the proletariat to fight depends in turn on its objective conditions. Revolutions, for good reason, tend to happen only after everything has gone to shit. They happen when the bourgeoisie, due to the falling rate of profit, no longer has the ability to provide ameliorating concessions--when even the most left-wing of bourgeois parties are no longer able to stave off revolutionary fervor. It is in these time periods that people genuinely feel they have nothing to lose.
It is tempting to believe that the pathway is to create a social-democratic government, and then start a revolution from that strong pro-worker position, where the government already owns most key industries, for an easy victory over the bourgeoisie. Yet, because social democracies are built specifically by the bourgeoisie to reduce the chance of revolt, a revolt against such a government would have no supporters, either within or without the government. Absent an extreme level of proletarian class-consciousness, the revolution will not happen so early. Perhaps in a world defined by socialism, where its successes were admired world-over, this would be possible: in the modern day, not so much.
This is the basic reason electoralism cannot result in revolution: at some point, you have to fight against the bourgeoisie, and all of their resources, and the resolve to do this only comes from a deterioration in objective conditions far beyond that which might drive someone to vote Socialist. Not only is it that liberal democracy cannot lead to a revolution, but a revolution can only happen when liberal democracy can lead to no improvements at all. The bourgeoisie must have no other options but to force us to revolt against them.
As Communists in non-revolutionary times, as we in the imperial core live in, our job is to make it so that revolution will happen faster, such that things do not have to become so awful. Ideally, capitalism can be overthrown before it causes an irreversible climate or nuclear catastrophe. Our job is to organize, educate, and inspire the proletariat, to push them forward, to train future generations of Communists, such that the people have the hope and strategy to win a revolution sooner. It also means resolute support for revolutions, and effective post-revolutionary states, that appear elsewhere.
Rule of the Proletariat
The only way to end capitalism is to forcibly take the means of production away from the bourgeoisie, and give them to the proletariat. The only way to do that is to create an apparatus with enough power and centrality to arrest power from those who have it and give it to those who don't. In short, the solution is to create a dictatorship where the whole of the proletariat is in charge.
But what makes this dictatorship proletarian? How can such an intensely central body be democratic? Why wouldn't it just be oppressive to everyone?
Democratic Centralism
Under capitalism, there exist irreconcilable differences between not only the proletariat and the bourgeoisie, but also different factions of the bourgeoisie. Thus, you must end up with a competitive system of governance, where only one set of interests can be represented at a time.
On the other hand, once you have taken power from the bourgeoisie, there are no longer any such irreconcilable differences. The only "classes" to be fighting one another are, hypothetically, the people and the government apparatus itself.
Within this framework, you have the average person, who wants a prosperous life, good government, and economic progress. Then, you have the government, which simply wants to remain in power. To do this, it needs to keep its people from revolting, and maintain a decent amount of economic progress.
The thing is... these two conditions are not mutually exclusive. A government whose only wish is to stay in power is in fact incentivized to do well by its people.
This is not always the case. If the people have no way of measuring the success of their government, if the government has a complete hold on information, then there is no need for the government to actually be good to its citizens. But a major component of remaining in power is being able to compete economically and militarily with other nations - and doing this requires losing the strict control you could possess over the means of communication. No amount of censorship can prevent word of mouth from spreading in large cities, and no amount of censorship can prevent people from forming online communication networks. And if people can do that, then social movements can spread, and revolts can occur.
A much more effective strategy to maintain power, in the long term, is just to be good at governance - to align yourself with the will of the people.
Lenin's greatest contribution to the development of Marxism is the mutually beneficial relationship between centrality and democracy. As-in: If you govern according to the will of the people, then trust in you will rise, and your power will increase. And if your trust rises, and your power increases, then you can better govern according to the will of the people.
Various mechanisms can be instituted towards this end. Consultative democracy is a great way of doing things: making decisions by having citizens air ideas and concerns across many forums, concentrating those ideas towards the top, and refining them into the best course of action. Grassroots democracy is another tool, where you let people design their own democratic systems, such that they all fit the local context, whatever stage of development they are in. A delegate system for the general electoral system is a good idea, also: it encourages consensus decision-making and allows experience and wisdom to be kept around for a long time. Cooperative democracy also means establishing public oversight over governmental processes. Above all, though, cooperative democracy requires experimentation: this is a new way of doing things!
This idea may sound foreign to people in the West, like you and me, but it is possible for a government to actually make decent decisions. It so happens we have had nothing but bad governance our whole lives--our good policy ideas are either ignored or treated as political footballs. But this is ultimately the case because of the class conflict--because the government's power to make positive change rests on whether or not said change would conflict with bourgeois interests.
But socialist governance does not have to happen this way. It may have to make tough choices, and it may perform grave errors, and it may carry on the bigotry that its people have not yet shed, but it can and will govern according to what it thinks the people want, and what it thinks will gain it legitimacy in the eyes of the people. It can do this because of the previous crushing of the bourgeoisie--because there are no longer any irreconcilable class differences in society.
This leads us to the organizational form for taking power in the first place.
The Vanguard Party
The concept of the vanguard party is that you apply democratic centralism to create an apparatus able to challenge the current state for power. I haven't actually studied this yet. Whoops!
The important part is just that it's a single body. Instead of having a bunch of trade unions carry out the revolution, or a messy conglomerate of interest groups, you have the Communist party. It's your only real option. This doesn't mean denouncing all ties to other organizations, but that when it is time for the revolution to actually happen, you need to minimize internal conflicts.
Anti-Imperialism as a Priority
Lenin's other greatest contribution to the development of Marxism was a fleshed-out understanding of Imperialism, or how capital uses nations and borders to its advantage. His ultimate conclusion for a revolutionary is that the imperial bourgeoisie must be kicked out before any attempt is made to kick out the national bourgeoisie. If you fight only against the national bourgeoisie, then you won't end up with socialism--you'll just transform yourself into a colony under the imperial power. Your first goal is to ally with the native bourgeoisie to kick out the imperial bourgeoisie--only then can you complete the job, and kick out the native bourgeoisie as well.
This places Marxists-Leninists as resolute anti-imperialists and anti-colonialists. The real independence of a country is a precondition for the liberation of workers in that country. So Marxists-Leninists care much about anti-imperialism, and anti-hegemony. Currently, this means opposing the US, and overseeing the decline of the American Empire.