American politics can be broken down into two categories: politics as coalition-building and politics as principle. Since the Ronald Reagan presidency, those two have been fused in the Republican Party more closely than they generally are, for so long that we take this to be normal.
But that kind of tight marriage between coalition-building and principle is never normal, because most voters simply aren’t that ideological. Their votes are determined by a mix of broad sentiment and narrow self-interest.
Ideological politics is necessary; without it, policy degenerates into apathetic clientelism. But ideological politics is never sufficient without a charismatic figure such as Reagan to divert the party’s focus from the inherent tensions of an ideology-driven coalition.