Here’s the key point: The 2018 elections did not make the Democrats a more left-wing party. It had the opposite effect: A large share of the new Democrats in the House hails from districts — many of them suburban — that in the past would have happily elected Republicans with moderate-to-progressive inclinations.
Such Republicans, once a substantial minority in the party, are a virtually extinct species. The rightward shift of the GOP began before Trump’s rise, and his extremism has, in turn, led to the defeat of even moderate conservatives. The survivors (with occasional brave exceptions) generally moved his way, fearing defeat in primaries.
This has had a peculiar effect on our politics: Many of the most important policy debates are no longer between the two parties; they are being carried out almost entirely inside the Democratic Party.