A fair warning: extrapolation of a current trend, however tempting and easy it might be, and however appealing the curve of the trend may be, is one of the riskiest predictions available. Just think of how the stock markets were doing in the middle of 1929.
Can cultural trends tear down the Democratic Party? They did in the 1970s and 1980s. Can a Democratic President push too far and too fast, inspiring well-heeled elites to finance a counterattack that forces some of the most reactionary politics ever upon America? That is exactly what happened.
Can Republicans push too far and too fast, alienating large numbers of voters? Stay tuned.
American politics implies coalitions of voters, some of those coalitions extremely fragile because of contradicting interests among members of those coalitions. The ascendancy of one Party over the other has typically involved one Party then in the minority poaching members of the current coalition of recent victories, members whose satisfaction with recent results is shaky at best.
To put it another way: Neither of the two major political parties wins or loses all the time.