Nowadays, the Democratic party is seen as more liberal and socialist and the Republican party as more conservative and traditionalist, but this wasn't always the case. There was a time when the republican party was the more liberal leaning and the democratic party were their conservative counterparts. When did this change and why? Was it an instantaneous change or was it a gradual process?

P. S: "switch ideologies" might not be the appropriate words to use since they're both advocating the ideology of democracy, but I'm not sure what to replace it with.