California Republicans suffered another blow on Thursday night as Representative Mimi Walters, who has represented Orange County for two terms, was defeated in her bid for re-election by Katie Porter, a law professor from the University of California, Irvine.

The Associated Press reported the result.

Democrats have now captured three of the four congressional seats in Orange County, a region that for a generation was a symbol of Republican power. The victory by Ms. Porter means California Democrats have won five of the seven Republican seats they identified as targets this year. Hillary Clinton defeated President Trump in all seven of those districts.

[In Maine, Bruce Poliquin, the lone Republican in the House From New England, lost his re-election bid.]

It appears the only successful Republican in the group is Representative David Valadao in Central California, who was declared a winner, though his lead over his Democratic challenger, T.J. Cox, has been shrinking as vote counting continues. The deadline for completing the count is Dec. 7.