The election in 2006 is a particularly relevant example, because Democrats had a somewhat similar, if better, set of opportunities. Those chances yielded 31 seats, just a few more than the 24 seats they need in 2018. But Democrats also had some good luck in 2006 that will be hard to duplicate: There were a half dozen safely Republican districts where the incumbent succumbed to scandal or indictment, including Tom DeLay, a House majority leader.

The Republicans have a real shot to retain control of the House in a political climate that would doom them under typical circumstances. There are a lot of reasons for this structural G.O.P. advantage, like partisan gerrymandering, the inefficient distribution of Democrats in heavily Democratic cities, and the benefit of incumbency.

To retake the House, Democrats will ultimately need to carry seats with a clear Republican tradition. This year’s special elections, including Jon Ossoff’s loss to Karen Handel in Georgia, are a reminder that it will indeed be difficult for Democrats to win in Republican-leaning districts, just as it was for the Democrats in 2006 or for Republicans on Democratic-leaning turf in 2010.

Image Supporters of Jon Ossoff watching election returns Tuesday night. Credit... Joe Raedle/Getty Images

The good news for Democrats is that they don’t need to win all of these Republican-leaning districts or even most of them. Democrats might only need to win, say, 17 of the 60 seats where Republicans are favored, but where Democrats have a realistic chance.

In that sense, these Democratic losses are entirely consistent with the possibility of a House takeover. If Democrats keep running ahead of expectations across those plausibly competitive Republican-held seats, many seats will ultimately fall their way. But they will certainly lose more than they win. The question is whether they win enough, and no special election offers the answer to that.