By the same measures, the 24th-most vulnerable Republican is Dana Rohrabacher, whose district is immediately north of Mr. Issa’s — stretching up the Orange County coast from Laguna Beach to Sunset Beach. In between, Ed Royce and Mimi Walters represent the 13th- and 20th-most vulnerable districts.

Of course, the exact House battleground will be shaped by a lot more than these few factors. Democratic recruitment and Republican retirements will play a big role, and a competitive race will expose the vulnerability or resilience of individual Republicans to a degree that recent elections have not.

But Orange County is not an outlier. Across the nation, the most vulnerable Republican incumbents among the 50 or so most competitive seats tend to be in relatively well-educated, metropolitan districts with above-average Hispanic populations. It’s the opposite of most of the 2016 presidential battleground states, which were whiter, less educated and far less Hispanic than the country as a whole.

Mr. Trump might still be riding high in central Pennsylvania steel towns, but there are plenty of signs that his support remains weak in precisely the districts where House Republicans are most vulnerable. The most recent Pew Research poll found that Mr. Trump had just a 38 percent approval rating among white voters with a college degree, with 61 percent disapproving. Mrs. Clinton probably won well-educated white voters by only a narrow margin, so the Pew result seems to imply a weakening in his standing.

Mr. Trump’s best poll of the month, from Fox News, had his rating among college-educated white voters at just 45 percent. Most Hispanic voters, unsurprisingly, remain deeply dissatisfied with the president as well.

Of course, Hispanic turnout is notoriously weak in midterm elections. There are several districts that look competitive on paper but might prove elusive to the Democrats if the electorate remains as old and white as it was in 2014 or 2010.

The well-educated, often traditionally Republican-leaning voters who supported Mrs. Clinton in 2016 will be a puzzle for both parties. Mr. Trump will plainly be a burden for some Republican politicians, who will agonize over how much to distance themselves from the president.