America isn't built for constant 1-party domination. I guess it's entirely possible (probably even likely) the Dems will win the majority of elections in the coming few decades, but I don't think it will become a borderline one-party state. In that case the Democrats would eventually become either too arrogant or too radical, and the GOP would adapt (though that might take some time with the recent radicalization of the GOP).
I think EASILY the most likely scenario is for Democrats to get a very, very significant governing and EC advantage by the 2040s that causes Republicans to move into a 1940s to 1960s position again of being the "measured, conservative, sensible" opposition to "radical liberal ideas." This will, of course, allow some inroads into groups they're currently offending. From there, it's anyone's guess how the GOP gets back to 1980s, 1990s and 2000s level strength again, but they will somehow. These two parties have made it through an AWFUL lot.