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.

So far, four Republicans have announced they will not seek re-election in highly competitive districts (with a Cook Partisan Voter Index of Republican+5 or more Democratic; the Cook P.V.I. measures district partisanship based on its vote in recent presidential elections).

They are Ileana Ros-Lehtinen (representing Florida’s 27th District), Dave Reichert (Washington’s Eighth), Charlie Dent (Pennsylvania’s 15th) and Dave Trott (Michigan’s 11th).