The process began back in 1964 when Goldwater celebrated extremism and denigrated moderation during the Republican national convention. But Goldwater was trounced electorally. And both parties had their wackos anyway.
What happened is that Republican craziness, crackpot promotion, civic well poisoning and bomb throwing ratcheted up starting with Nixon, while the left started seriously self policing after Mondale got trounced in 1984. In 1992 there were signs that something was seriously wrong with the GOP, but in fact the saner elements generally had a firm upper hand. Those who governed figured they had to throw red meat to the dogs every election year (in Nixon’s formulation) but that was treated as just the cost of doing business. Sure, Reagan had rhetorically boosted supply side crackpottery and formed close ties with anti-science evangelicals. But at bottom he was an ideologically hard right President who was willing to compromise for his ends. That matters.
The inflection point was Newt Gingrich during the early 1990s. Deciding to shut down the government in order to blackmail his way over a Presidential veto threat betrayed a fundamental misunderstanding or rather indifference to the habits of compromise that form the basis of the US democratic system. The fact is, systems formed on division of powers are susceptible to gridlock, which is why Parliamentary systems have a better survival rate in the world.
The 2nd development was the early 1990s rise of the conservative electronic media, which didn’t care too much about getting their facts right. It’s all emotional appeals, all the time.
Back in 1960, Republicans had an absolute lock on the professional class. Now they are anti-science and anti-scholarship: the level of debate is just sad. It’s no surprise that the professional class is trending Democratic, notwithstanding their affluence.