Slate, “Democrats Haven’t Gone as Far Left as Republicans Have Gone Right”:
Today’s GOP is more conservative than any party formation in 100 years, versus today’s Democratic Party, which is only modestly more liberal than it was during the Clinton administration.
Among Republicans [polled by Gallup], 70 percent identify as conservative. By contrast, just 43 percent of Democrats call themselves liberals. It’s a substantial shift from the recent past, but nothing like the GOP’s conservative supermajority. Different data, from the Pew Research Center, tells a similar story.
And don’t forget the other big factor in consolidating conservatives firmly under the Republican banner and outside the Democratic ranks: The “polarization” resulting from segregationist / southern conservative Democrats quitting the party to join the Republicans in a few big waves in the past half-century.
Even just in the past few years, losses for Democrats in Congress came heavily from among conservative Democrats in the South (and the Mountain West to some extent), making the remainder more liberal only by subtraction, while noticeably strengthening the ranks of conservative Republicans.