VISTA, Calif. — Representative Darrell Issa was 2,500 miles away from his Southern California home the other day, on a fund-raising mission to Florida. Outside his congressional district office here, a light drizzle fell from the sky. It was a weekday morning.

But no matter. Close to 1,000 people were gathered for yet another weekly demonstration, a jumble of umbrellas, placards (“Repeal and Replace Issa”), honking horns and angry chants. They denounced Mr. Issa, a Republican who has held his House seat since 2001, for his support of President Trump and the Republican health care bill.

“There are a lot of people who are very passionate,” said Nancy Arwine-Mavers, 58, a cardiac sonographer. She said this was her first political rally since marching for the Equal Rights Amendment in the 1970s. “People who usually don’t protest are coming out. They are realizing that one vote could cause them to lose their health care.”

If Democrats have any chance of capturing the 24 Republican seats they need to take back control of the House, the road to victory starts here in California, and particularly in Orange County, a former conservative bastion that favored Hillary Clinton in 2016. It was the first time the county had voted for a Democratic presidential candidate since Franklin D. Roosevelt in 1936.