I'm not saying it can't happen, but recent history has shown that all of the most authoritarian leftist regimes around the world arose out of and in response to (1) an extremely repressive and conservative monarchical/feudal system, or (2) an extremely repressive and conservative colonial system. I don't think any have arisen out of a liberal democracy. That doesn't mean it can't happen, but I think the fact that it hasn't so far is pretty important.
As another matter, the Constitution didn't even imagine political parties. So I'm not sure what you mean when you say we'd have to throw out the Constitution.
I agree however, at least in the long-term, that to flourish any organized society can't be controlled by one faction or party. People disagree about shit, so they need a system that enough of them recognize as legitimate to mediate their disagreements. Even if (like now) one side is clearly right about just about everything and the other side is a serious threat to the very system itself, that doesn't mean the side that is right will always be right. A one-party system ossifies and after awhile is unable to address the needs of most people.
But that's a long-term issue and shouldn't be confused as indicating that we need two competitive political parties right now. Right now, the political will of the American right needs to be splintered and destroyed.