The president had a 73 percent approval rating among California Republicans in a poll last month by the Institute of Governmental Studies at the University of California, Berkeley, which, while high, was still considerably below what Barack Obama enjoyed among Democratic voters when he was president.

Mr. Trump and congressional Republicans have championed a tax bill widely assailed as punishing to California homeowners, dismantled policies aimed at fighting climate change and pushed tough and restrictive immigration policies. Last year, Californians voted to legalize recreational marijuana by a hefty margin of 57 percent to 42 percent; days after the law took effect on Jan. 1, the Justice Department announced it would aggressively enforce federal strictures against the drug.

The Interior Department moved last week to allow offshore oil drilling, a policy that has a long history of bipartisan opposition among Californians. The next day, it exempted Florida from the policy in response to pleas from Rick Scott, the Republican governor running for Senate, drawing accusations that the Trump administration was punishing this state.

“Most days for California Republicans in Congress are days spent on defense,” said Bill Whalen, a research fellow at the Hoover Institution and a former senior adviser to Pete Wilson, a Republican governor. “Defense spent on why Donald Trump wants to pull the country out of the Paris accords. Defense trying to explain why the president won’t give the same offshore drilling waiver to California as it did to Florida. And defense trying to explain the latest thing Donald Trump said that he’s now denying that he said.”

Mr. Chávez said he would not have voted for the tax bill and opposed the tough immigration policies backed by some Republicans in Congress. “Offshore drilling — I don’t support it,” he said. “I’ve been in California my whole life. I remember the Santa Barbara spill.”

For all that, Republicans have some reason to hope they can withstand any Democratic wave. The fact that Mrs. Clinton won districts represented by these seven Republicans does not mean that voters will necessarily support a Democratic candidate for Congress in November. Although Republican enrollment in this state has been on a steady decline, the party continues to have a registration edge in these districts. Historically, Democratic turnout drops in nonpresidential years — especially in California.