Suburban Stragglers

The suburban stragglers, on the other hand, haven’t run so far ahead of their party. Many of these representatives were elected for the first time last year or the cycle before. Others have been around for longer, like Leonard Lance of New Jersey, but didn’t outpace their party’s recent presidential performances by an impressive amount.

And then there’s Tom MacArthur of New Jersey, who brokered the House health care compromise. If he had stayed out of it, his 21-point victory in 2016 would have kept him out of this category.

The White Working Class

Democrats have understandably focused on how they might be able to win back white voters without college degrees since Mr. Trump’s surprising victory in November. But when it comes to control of the House, the Northern, white working-class voters who put Mr. Trump over the top can’t take center stage.

It’s not because Democrats can’t win those voters. It’s because there just aren’t many competitive districts in white working-class areas. That’s in no small part because states like Wisconsin, Ohio and Michigan are so heavily gerrymandered that there aren’t many competitive districts at all.

Instead, most of the top Democratic white working-class opportunities are in states like Iowa and Maine, where the political geography doesn’t allow for a brutal gerrymander, or in states like New York and Illinois, where Democrats have more control. These districts will still be quite competitive, despite Mr. Trump’s gains, much as the Sun Belt districts will be tougher for Democrats than Mrs. Clinton’s results alone imply. Making matters a bit easier for the Democrats: Many of these representatives have been in office only since 2014 or 2016, and they did not run well ahead of Mr. Trump.

Weaker Than They Should Be

There might not be many top-tier Democratic opportunities in white working-class districts, but in a wave election the Democrats could become competitive in a broader set of those districts. One group that might come into play: the incumbents who seem to fare a little worse than they ought to.

Mr. Trump and Mitt Romney won these districts comfortably, but these representatives tended to run no better — or even worse — than the president. Even more concerning: Many of them, whatever the reason, have been underperforming their party for a while. Democrats might be slightly more optimistic about these seats than they would be about a typical Republican district.