There is no US in their worldview. It would cease to exist. Would it merge with Canada and Mexico? Who knows.
The constitutional form of government we have would be overthrown. With that, the constitution itself.
That is no "leftist US" any more than independent Russia, Ukraine, Poland, etc. are still the USSR.
The lack of independence among USSR states was in large part a result of their capture and rapid subjugation by Stalin in WWII, and AFAIK had little to do with the supremacy of their ideological unity.
I think that only a small minority of anti-war people want dictatorial power. But perhaps its a majority among their leadership depending on how thats defined. My sense is that most are just emotionally disturbed, misguided and acting out and want the easy answers and salvation that the left promises to the harsh realities that they cant or wont face. I suspect that most would love a far left version of the US, with a trendy progressive interpretation of the Constitution that protected them, even if it meant our standard of living went into freefall, but with them immune of course.
Its those people that Im concerned about because their message is persuasive. I think that the hard core US haters are generally counter productive and self destruct, as long as theyre forced to expose themselves like this.