It is a bit misguided to cast this as a Republican/Democrat issue, since the origins of it go back before the Southern Strategy, where a large cohort of historically Democratic southern segregationists switched to the Republican party, reversing the conservative/liberal poles and laying the foundation for the modern (Rightist) GOP.
The US has been on a protracted slide towards social-liberal values since the late 1950s, in part due to post-WW-II ideology which held the US as the primary source and protector of (small 'd') democratic values for the world at large. Put simply, the idea that the US protected the weak against oppression solidified in the American consciousness, and that attitude contributed to the successes of the Civil Rights movement, Feminism, gay rights, union movements, and other ostensibly leftist (social-liberal) projects. Conservatives — or more specifically, fundamentalist Christians, economic (free market) adventurists, and white ethno-nationalists, all for very different reasons — have been fighting rear-guard actions against these sociopolitical changes, trying to preserve elements of the 1950s social order. They all pulled together under the GOP 'Big Tent' sometime in the 70s or early 80s (certainly by the era of the Moral Majority movement), despite having little in common except their opposition to progressive social changes.
It isn't that the Democrats have been pushing the nation to the left. The nation as a whole has been moving somewhat left: towards a more secular, technological, cosmopolitan society. The Democratic party has followed that trend while the GOP has steadfastly rejected it.
Up to this point, we see the normal push-me-pull-you struggle between progressivism and conservatism, a struggle that normally slows down change to a manageable level without really stopping it. However, the modern aggrieved political polarization is best described as an advancing decompensation on the fringes of the far Right over their repeated failures to stop or reverse these social changes. I mean, there are always those who decompensate in society, effectively losing connection with reality and retreating into an enclosed bubble where they can maintain an alternate version of reality. Think the Branch Davidians, various New Age cults, the isolationist militia movements of the 80s and 90s, the odd and sundry revolutionary groups that have popped up over the years... But beginning with the collapse of the HW Bush administration (under the mortgage lending crisis) and the nomination and then election of an African American president, large segments of the far Right lost their footing in reality under the stress, insisting that how they viewed the US was reality, and that the Democrats and liberals must have done something horrible to make the US not fit the way the Right perceives it. Add the FOX Network's propaganda paradigm and the tendency of social media to fracture into like-minded enclaves, and perfect conditions were created for a persistent, globally-scaled decompensation.
In short, the far Right now insists that their perspective is true and right, even if it involves Satan worshiping pedophiles, Jewish space lasers, massive undetectable election fraud, and secret nefarious plots lying behind a global pandemic. And they've reduced themselves to obstructionist, unfocussed guerrilla tactics. It isn't that they've 'maxed out' on Rightist ideology, but more that they've lost themselves in such a thoroughly alternate reality that conventional Rightist ideology is too much part of the real world that they are rejecting. They cannot be old-school Rightists without acknowledging that the ongoing social changes are (small 'd') democratically legitimate, and they cannot accept the legitimacy of these changes. So conventional Rightism has to go with all the rest.