Taking the long, historical view, a peculiarity of U.S. politics before 1980, was that ideology and party were largely divorced. In the 1950's and 1960's, there were liberal Democrats and liberal Republicans. In the first two decades of the 20th century, there were progressive Republicans and progressive Democrats. The State parties, the national (Presidential) parties, and the Congressional caucuses, were, at most, loose, overlapping alliances. In the 19th century, the political parties were founded on patronage, and ideology was a distant, secondary consideration for most of their members; the State parties tended to be ideologically cohesive, but not the national parties, creating a regionalism, which cross-cut philosophical outlooks. Ideological movements, like abolition, prohibitionism, populism and progressivism, the civil rights movement, evolved outside the parties; labor unions made only distant alliances with the parties. Congress, generally, overpowered all but the most vigorous and domineering Presidents, and in Congress, seniority (and in the Senate, individual privilege) dominated party loyalty in creating power.
The advent of ideologically driven Parties, at the national level, is very recent in the U.S., occurring only after 1980. The Republicans have led in this, and have had ideologically pure control of the Senate only since 2002. The final remnant of 4 "moderate" Republicans, is less than the five vote margin. The Democrats have a somewhat larger "conservative" contingent in the Senate, but, in the House, both caucuses are pure enough that the Republican majority is also a solid conservative majority. Reagan had to cultivate conservative Democrats and Clinton could still manipulate moderate Republicans, but those days are over.
Because ideological "purity" is so new, neither members of Congress nor voters, have as much experience with it, as do polities, with longer traditions. Although the U.S. looks stable, it may be in a state of profound disequilibrium, in which parties, politicians and voters change their behavioral strategies radically, seeking some kind of, as yet unrealized, Nash equilibria.
The current dominance of conservatism, nationally, is a path-dependent result of how the parties have evolved, especially in Ohio, Florida, Texas and the western Mountain States. In Florida and Texas, conservative Republicans and conservative Democrats have long competed for conservative voters; with Democrats having lost that game, the Democratic parties in those States may veer left seeking new constituencies. In Ohio, Republican control is tenuous. In the western Mountain States, Democrats may succeed with an oddball progressive, libertarian philosophy.
My bet would be that the Republicans have secured themselves a kind of authoritarian, Imperial rule, and the Republic is dead, the Congress, a eunuch, and the Constitution has become exactly the dead letter, the most conservative Republican judicial appointees want it to be.
But, my hope, however, faint, is that the Republican imperium is built on sand, and may be swept away, in the wake of the disasters their incompetence will inevitably visit upon us.