system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When and why did the US political parties "switch"? Ex. Republicans in early 1800s being generally progressive and Democrats being generally conservative, but now it's the opposite.
gpt
The switch started in the 1940's when the Democrats officially adopted Civil Rights as part of their party platform. What really kicked it into gear though was the 1960's.

After JFK was assassinated and LBJ became President, he rammed the Civil Rights Act down Congress' throat. This was the first real sign to a lot of voters that Democrats were now interested in progressive values. 

In the 1964 Presidential election, Republicans reacted by nominating Barry Goldwater, a very conservative candidate. He lost in one of the largest landslides in American history. The Republican party had to do some soul searching, and the hard-core conservatives seized the party power positions and started using populism to win elections. By scaring white voters about crime and civil rights and the end of morality in America, they could win elections. Ronald Reagan was swept into the CA governor's mansion on a wave of populism. The result is a more conservative Republican party.

Meanwhile, northern liberal Republicans are getting ostracized in their own party (like John Lindsey) for supporting civil rights and voted out of office by voters for not being Democrats. 

At the same time, northern Democrats are going harder for civil rights and voting rights while southern Democrats feel abandoned by their party and are starting to flip to Republicans  (like Strom Thurmond) or going rogue (like George Wallace). 

At the end of the day, the crazy cultural liberalism of the 1960's drove a huge conservative backlash that found root in the Republican party and turned it more conservative.