Nate Cohn for NYT’s The Upshot:

Democrats often lament that the House electoral playing field is stacked against them, but that’s not their only problem.

The Republican structural edge in the House is fully realized only with the added advantage of incumbency. The Democrats would be overwhelming favorites to retake the House in this political environment if no incumbents chose to run for re-election, even with all of the burdens of gerrymandering and geography.