Just because Tuesday’s victories came in states or districts won by Mrs. Clinton doesn’t mean they can be dismissed, however. College-educated white voters, paired with nonwhite voters, could profoundly endanger the G.O.P. in traditionally Republican, upscale districts.

Up until the polls closed in Virginia, it was reasonable to suppose that Mrs. Clinton’s performance represented something of a ceiling for Democrats in well-educated areas. It would be easy, the theory went, for Republican voters to distinguish their longtime Republican representatives from Mr. Trump. Jon Ossoff’s failure to outdo Mrs. Clinton in Georgia’s Sixth Congressional District seemed fully consistent with that possibility.

But well-educated voters in Virginia didn’t appear to make any distinction between Mr. Trump and their incumbent representatives. Ed Gillespie, who was thought to have run a pretty strong campaign for governor against Ralph S. Northam, wound up running well behind Mr. Trump in many well-educated suburbs, a possibility that few imagined heading into the contest.

The catch, though, is that the overwhelming Democratic strength in well-educated areas did not cross the political divides of the 2016 election and reach into white working-class areas. In fact, Mr. Northam, a Virginia Military Institute graduate with a strong Southern pedigree, didn’t even come close to matching Gov. Terry McAuliffe, Mr. Obama or Senator Tim Kaine in rural western Virginia. Democratic State Assembly candidates didn’t run well ahead of Mrs. Clinton, either.

Yes, the political divisions of the 2016 presidential election wound up working pretty well for Democrats in Virginia, a highly educated state. But that might not be the case for Democrats in a lot of the rest of the country. There are only 11 Republican-held congressional districts in the United States where Mrs. Clinton won by five points or more. Even if Democrats swept those 11 districts, it wouldn’t get them that far toward the 24 seats they need to flip the House.

To my surprise, it’s not obvious that a rerun of the Virginia House of Delegates election on a national scale would yield Democratic control of the House. Without greater strength in areas that supported Mr. Trump, it would still be a tossup.

The good news for Democrats is that they did run well ahead of Mrs. Clinton in white working-class areas during this spring’s special congressional elections. And on Tuesday, Mr. Northam ran ahead of her in some of those areas, too, even if he landed short of prior Democratic benchmarks.

The big question in 2018 might prove to be whether Democrats can have it all: Will it be possible to combine a Virginia-like near sweep of Republicans in Clinton districts with a broad Democratic overperformance in white working-class districts? If they can do both, they will be favored to retake the House. One or the other would probably make the fight for House control a tossup. The fact that they’ve done both at various points this year might be an early clue.