A very good comment. I would add that the parties used to be more moderate in part because the Democrats included both Northern liberals and Southern racists, a

A very good comment. I would add that the parties used to be more moderate in part because the Democrats included both Northern liberals and Southern racists, and the Republicans included many Northern liberals. But when the civil rights laws were passed by the Democrats back in 1964 and 5, Southern white racists started a slow process of migrating to the Republicans and the Northern liberals to the Democrats. Also, changes in the primary system lead to party leaders, who tended to nominate moderates who could win a broad range in the middle, losing power, and the nominating process being taking over by people on both extremes who tend to vote more in primaries than the general public does.