I don't know if it's a lifetime of brainwashing by Republicans and Evangelism,
From my point of view and experience, the downhill slide began in earnest with the election of Reagan in 1980. The myth of the “Moral Majority” was perpetrated as if there existed a catch-all category of Americans who believed similarly on a wide range of subjects, as Nixon had also done a decade earlier by invoking his “Silent Majority” supporters.
Reagan embraced the notion and behaved as if there was a mandate from “his” constituency to bring about a plethora of radical quasi-fundamentalist “religious reforms” many revolving around sexual and reproductive issues.
At that point the flagship issues of abortion rights and “gay liberation” were generally accepted as settled doctrine around the country, certainly by nearly all young people and/or progressives. For example, abortion was a 3rd-tier social issue that was all but insignificant in the 1970s, and gay people were pouring out of the closet with little fear. Reagan’s dramatic lurch hard to the right was only made possible by what Jimmy Carter referred to as the country’s “malaise” in his famous 1979 speech.
https://www.washingtonpost.com/news/wonk/wp/2013/08/09/jimmy-carters-malaise-speech-was-popular/?utm_term=.0e81d297d1a0From my experience, the national mood of the US peaked in the late 1960s as our decade-long space race, in which we were all emotionally invested, and climaxed with televised Americans walking on the Moon. Euphoria is hardly a strong enough word for what we felt, and I feel certain that this country, as a whole, has never experienced anything like it, before or since. Generally, epic celebratory events result from the winning of wars, worthy accomplishments for certain, but hideous and gruesome nonetheless. Americans walking on the Moon was pure true human accomplishment.
Immediately afterwards, we suffered the double whammy of facing the ugly conclusion of the Vietnam war and then the humiliating demise of the Nixon presidency. (note that while I think that Nixon was a pathetic and disgusting person, he was not a bad president, as those things go)
This left the country fearful and disillusioned, and the James McGill Buchanan’s stealthy radical-right organization sprang into action. By co-opting what Reagan considered to be his “Moral Majority” under the funding of the ultra-wealthy tax-haters, the hard right began its inexorable march to the Trump horror.
Make no mistake – the one and only purpose of everything that the Republican Party does today revolves around stripping the meager wealth of the working classes and transferring it to the already-ultra-wealthy. Social issues such as the traditional “3-Gs” - God, guns, and gays are meaningless wedge issues drummed up to the forefront to motivate the ignorant to vote directly against their own self-interests.
Simultaneously, Carter’s malaise infected the center and left with increasing despair, and since there was no corresponding organizational impetus than to simply strive for the “common good” that the Founding Fathers trusted the people to steer towards, progressives have fallen into lethargy for a couple of generations.