The thing with the "parties as political bloc of varied interests" situation means they don't stand up well if given control over the government with little opposition: They start splitting to their seams over all the different things they want, and eventually they collapse under their own weight or start shedding interest groups which form separate parties and die and cost them votes; either way
letting the other party return to grace, having shed fat in the form of unpopular policies, politicians, or other undesirable traits and ready to fight the good fight. This takes the form of anywhere from being mildly preferred to a party on the verge of total control, but still eventually returning to equilibrium.
It's happened before. A lot. In both directions. In a short term the system is unstable, but from a long view it eventually works out evenly. Of course, if MSH is right and electoral reform occurs under a Democratic majority (which is entirely possible depending on how long and how complete their control is; the republicans are losing a serious demographics war right now, and democrats pass a lot of cool things when they have total power) then no one can predict what will happen, but likely the end of American politics as we know it (for better or for worse).
Also, if it becomes 90% blue, what's to stop them actually trying to keep the populace from voting for the other party?
They control the vast majority, they can screw over the people more often.
I'm not sure if they'd really become a totalitarian state like that, that's a bit far fetched, but I do fear a system where the Democrats have that much power. Them "screwing people over" isn't all that far fetched, especially if you end up with someone like Hillary who's a bit too authoritarian for my liking.
I'm not saying they go all-out insane, but they could afford to start passing unpopular legislation because it benefits themselves.
greatorder, If they somehow managed a 90%, combining the will of the conservatives, the libertarians, the liberals, the radicals, the everything, (bearing in mind 20% still believes Obama is a muslim, and about half the self-declared democrats positively view socialism) into a single party,
then maybe they should be allowed to pass whatever the hell they like. I think that's a mandate of the people If I've ever seen one. The 10% of the population is almost certainly the extreme segment. Any dictatorship with steady 90% support is a dictatorship of the people (hah, I joke).
But 90% isn't even necessary, 60% is. With that, a so-called "super majority", Congress can pass whatever it wants and there is no way the minority can really stop it. There have been three of these in the modern era, and one of these was in 2009, which should give you some insight into how long they last. That particular congress passed The Affordable Care Act, actually, which should also let you know about what happens when they pass unpopular legislation (the earliest one passed the Voting rights act, Medicaid/care, Freedom of Information Act, and things of that nature; the 95th was Carter, which speaks for itself (although he passed the Clean Water Act, which is a plus)). You know, they say the majority and opposition party in American politics are like the sun and moon: the moon "shines by the light of the other". Well they do that pretty damn well.