Republican-held districts that Clinton won in 2016 California, a liberal state that has repeatedly clashed with the Trump administration, features several races this year that Democrats hope to flip to win back control of the House. Of the state's 53 congressional seats... ...only 14 are held by Republicans. Seven of these districts favored Hillary Clinton in 2016. These are the districts Democrats think they have the best chance to flip. But it remains an open question what the presidential result may mean for Democrats in the midterms.

District Rating 2016

Presidential Margin 2016

Congressional Margin 49th , Darrell Issa * Tossup 10th , Jeff Denham Tossup 25th , Steve Knight Tossup 39th , Ed Royce * Tossup 48th , Dana Rohrabacher Tossup 45th , Mimi Walters Lean Rep. 21st , David Valadao Likely Rep. *Incumbent is not running for re-election.

While it might seem surprising that these Republican-held districts voted for Mrs. Clinton, it did not come as a shock to longtime observers of California politics.

“These have always been districts that are on the cusp of some kind of flip,” said Eric McGhee, a research fellow at the Public Policy Institute of California who specializes in the state’s political geography.

Though it is unclear if the vote for Mrs. Clinton was anything more than a protest against Donald J. Trump, Mr. McGhee said, “Certainly the partisan climate is as favorable as it’s going to get for Democrats.”

The 2016 outcome was more about “depressed Republican turnout and heightened Democratic turnout” than “some major shift in partisan preferences,” according to Mark Petracca, associate dean for the School of Social Sciences at the University of California, Irvine.

Nearly every district shifted strongly Democratic in the 2016 presidential election.

Four of the seven districts represent parts of Orange County in Southern California, where more traditional Republicans — limited government, free-market conservatives — could not bring themselves to vote for Mr. Trump, he said.

Republicans also turned out in lower numbers in 2016 because of a “lack of exciting candidates” in down-ballot races, Mr. Petracca said. For example, both candidates for the open United States Senate seat were Democrats.

And then there are the two Central Valley districts, in the state’s agricultural middle, where voters already had a history of favoring the Democrat at the top of the ballot but not those lower down.

To be sure, there are some long-term demographic changes that have given hope to Democratic candidates.

In particular, northern Orange County has become more diverse, with growing Asian and Latino populations, and is notably more liberal than the southern part of the county.

But the real story in Orange County, according to Mr. Petracca, is not that the Democrats have gained voters, but that Republicans have lost so many — and Democrats have failed to pick them up.

“Republicans have lost a major chunk of the voter registration pie and Democrats have picked up a little. The real ‘winners,’ as it were, have been the Decline to State voters,” he said, referring to California voters who register without choosing a party affiliation.

The state’s unorthodox “top two” primary system threatens to get in the way of whatever chances Democrats have in some of these districts.

Under the system, all candidates appear on the same primary ballot and the top two winners advance to the general election in November, regardless of party.

This year, Democrats may have the best chance to win in the 39th and 49th Districts, where the Republican incumbents are retiring.

But because the Democratic field is so crowded in those races — and there will be at least two viable Republican candidates without an incumbent on the ballot — votes for Democrats could be divided too many ways, preventing any of them from finishing in the top two.

Since the new primary system took effect in 2012, there’s been one House election where the two candidates did not reflect the district’s partisan lean. That year, Pete Aguilar, the Democratic front-runner, took third place behind two Republicans in the primary for a new congressional district that was expected to go Democratic.