A future disciplined vanguard party will serve the role you are talking about.

Would the correct illustration be,

Leftist party leads workers in organizing
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Leftist party grows stronger at negotiating with workers being an organized expression of solidarity behind them
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The capitalist police state resists. I assume the police state is capitalistically empowered and enforced, but not capitalist in itself. Meaning it won’t sanction the leftist party, but it will use taxpayer funds investigate, legislate, harass, and sell propaganda which advanced its agenda.
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Worker solidarity survives a bureaucracy that, in its default state, is procedurally guiding you toward any condition where you have less autonomy. Forms a disciplined vanguard in cooperation with the left to further resist the state.
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Leftist party demands legislative change that would actually threaten the status quo, e.g., demanding an end to the Iran war, or demanding Buckley v. Valeo be amended to set spending limits on candidates, getting rid of dark money, etc.
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The capitalist state refuses, rejecting the authority of any such party. Declares the vanguard a terroristic entity, investigates its members, threatens to sanction the leftist party if efforts continue. The argument is, you ought pass you legislation the old fashion way.
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A voter revolution, by electing someone from the Leftist party to pardon the vanguard members and issue legislation to protect worker solidarity.
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If the vote fails, rough… if it doesn’t, the pardons pass but the legislation gets locked up in one of the best mannerisms of preventing change the system has: the system itself. The president tries forcing it through with tactics like Executive Orders, but that just fuels Rightist propaganda that the socialists are taking over. They’ll get the Trump defects too, claiming this is what Trump tried for fascism.
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The right forms their own vanguard, largely with internal help from the military, FBI, and other entities. Their goal is to “preserve democracy” or whatever. To them, the Leftist party is threatening communism — something they believe leads to dictatorship.
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Both sides believe they’re doing the right thing, adamantly believing the other is wrong. The risk of civil war is high. The president is seen with the responsibility of preventing civil war, but also knows this is his one shot to enact change. They probably shuffles into a lame duck position.
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The economy remains functional, which helps stave off a civil war. The president has accepted the pardons and recovery of the left vanguard as a win. To include having prevented executive action against the leftist party. The president is credited with having avoided a civil war for this long, though mostly by not stirring the pot much. Legislation remains locked up, but that’s considered the least of our worries by election time and we get them a second term.
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The second term is mostly a repeat of how the first one ended. Civil war concerns have died down quite a bit, though both vanguards still exist. Some on the Left have grown complacent, some frustrated with the feeling that nothing has actually changed.
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By the end of the second term, the Right has promoted some figure as The Man to end your woes by ensuring we go back to how things were. They’ll speak of traditional values, simpler times, and foreign threats that we should stay unified against. They’ll get elected.
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The three branches of government are suddenly working fluidly again and President The Man finds some novel way to fuck up the economy. We now have our traditional political problems, but with opposing vanguard infrastructure and knowledge at the ready.
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Individual states begin organizing support for one another, forging relationships to protect themselves with the aid of whichever vanguard their politics are aligned. There are three main factions to consider now: the left, the right, and the weakening federal government.
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The Left eventually do something to piss the Right off. Probably withholding taxes, moving their economic assets / supply lines out of red states entirely, or maybe it’ll be water wars by then… maybe the threat of secession.
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The President, aligned with the Right, try enforcing their will in the situation. The Left use their vanguard to prevent such measures. We’re perhaps one shot fired away from full on civil war.
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The second American Civil War occurs, because why wouldn’t it in conditions like that?
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God knows who wins. The rest of the world will move on by then. Western allies will hope we don’t turn out like Russia. Foreign adversaries will be hoping we destroy ourselves in this process. Who knows who intervenes and how…