“Republicans are going, ‘Are you Trump or are you not Trump?’ ‘Are you this or are you that?’” she said. “You know what? This is no time to worry. Just get out and vote, and vote for the party, people, because the other side has gone way far left and it’s going to continue.”

The district of red-roofed Spanish-style McMansions and oceanside golf courses stretches from southern Orange County to northern San Diego County and includes the Marine base at Camp Pendleton. The crowd cheering Ms. Harkey included several veterans and a woman with a sparkling flag pin. A leader of a local Republican women’s club noted proudly that her red dress was from Ivanka Trump’s label.

As Ms. Harkey called out to make sure she had not missed any local officials — “School board, water board, anything else?” — a man in the crowd replied, “I’m all for waterboarding!”

Ms. Harkey told her story of arriving in California as a young girl from Illinois and having to peel onions with the Mexican bracero workers to earn $11 for her first Barbie doll. She went to community college and the University of California-Irvine, working her way up in banking and to become mayor of Dana Point, a wealthy oceanfront enclave. She served six years in the California State Assembly and is now a member of a board that reviews state tax assessments.

After nearly two decades of voting Republican in presidential races, the district went for Mrs. Clinton by 7 points in 2016. And Pat Bates, a state senator, warned the crowd at Ms. Harkey’s headquarters that Democrats had “overperformed” in their voter registration efforts in the June primaries.

“We need boots on the ground,” she said. “Every single Republican has got to vote or we’re looking at a socialist southern Orange County and north San Diego, I kid you not.”

The women at the Flip the 49th headquarters, who worked hard for that overperformance, began their activism out of concerns about the Trump administration dismantling the Affordable Care Act. They held weekly protests outside the office of Mr. Issa, a leading tormentor of Barack Obama and Mrs. Clinton, and once brought a cake in the shape of a Hawaiian shirt to encourage him to retire early; they returned to celebrate the next day when he announced he would.