Gerrymandering is not the only reason Democrats are at a disadvantage. Republicans also have the advantage of incumbency, which, on average, allows members to run about seven percentage points ahead of the national party.

But Republicans have gradually been losing the advantages of incumbency as well, most obviously because of 34 recent retirements in Republican-held congressional districts.

Over all, the number of G.O.P. retirements in plausibly competitive districts isn’t extraordinarily high. But some of the Republican retirements have been especially damaging: longtime incumbents who have a tradition of running far ahead of the national party and dissuading strong challengers, like New Jersey’s Frank LoBiondo or Pennsylvania’s Charlie Dent. Their retirements could easily be the difference between a non-competitive race and a Democratic victory.

The Republican incumbency advantage has diminished in another way: Democratic recruitment and fund-raising. A strong Democratic recruit — like a military veteran or an elected official — can cut into that advantage, especially with strong fund-raising numbers.

As measured by biographical indicators like military experience or past elected officials, Democratic recruitment is only somewhat above average at this stage. But much like the Republican retirements, the best Democratic recruits have often been extremely valuable.

Often, Democrats have succeeded in finding their very best candidates in white working-class districts where President Trump fared well in 2016. His strength threatened to move many traditional battlegrounds into the Republican column. The Democratic path to a House majority arguably looked even more difficult after the 2016 presidential than it did before — even though Democrats picked up seven seats in that election, since so many traditional battleground districts swung hard for Mr. Trump.