The retirements are so consequential because they deprive the party of the advantage of incumbency. Even in wave election years, incumbents generally win re-election in districts that tilt somewhat toward their party. Most Republican incumbents represent such districts, in part because of partisan gerrymandering and the tendency for Democrats to cluster in urban districts and “waste” votes.

It’s a different story without the advantage of incumbency. In a wave year, the president’s party struggles to retain open seats in competitive districts, even those that tilt toward their party. Often, a retirement is the difference in whether a race is competitive at all.

Here’s a rule of thumb: A Republican retirement in competitive districts costs the party about one-third of a seat, on average, in this political environment. Mr. Ryan and Mr. Ross were the 18th and 19th such retirements for the Republicans. Collectively, it suggests the retirements will cost the Republicans the equivalent of around six seats. (Of course, there are always such retirements, but the pace of them in this cycle is well above the norm.) That is one-fourth of the seats the Democrats need to retake the House.

It makes it increasingly difficult for the Republicans to prevail in a wave election.

One way to think about it is in terms of the national popular vote: What margin of victory would Democrats need to take the House? In 2014 and 2016, the Republican structural advantage was so large that the Democrats might have needed an eight-plus-point victory — even larger than the Republican margin in 2010 and 1994, and about as large as the Democratic margin in 2006. So it seemed that the Republicans had a plausible and even a good chance to survive a wave in 2018.

But today, Democrats would be favored to win with less than a seven-point popular vote margin. (This figure leaves aside the question of uncontested Democratic seats.)