In contrast, California alone could end up with eight or nine battleground districts.

“There’s no question we’re going to end up with more competitive seats,” said Rob Stutzman, a Republican consultant in Sacramento.

The first draft of the map shocked much of the California delegation. No longer able to count on his rural, agricultural base, Mr. Nunes would have had to win over the gracious neighborhoods along Van Ness Avenue in Fresno, with their verandas and Black Lives Matter flags, and the hipsters of the city’s Tower District, who have more affection for Devin Nunes’ Cow, a Twitter account mocking the congressman, than the man himself. The commission appears intent on giving Latinos in the Central Valley a chance to elect their first representative ever.

Mr. Nunes could have moved to a new district taking shape along the Nevada border, which will be heavily Republican, but he chose to go elsewhere. He was not alone in pondering a new future. After losing his San Diego-area seat to a Democrat in 2018, another outspoken conservative, Darrell Issa, moved to a conservative district abandoned by the indicted Republican Duncan Hunter. That seat could end up far more competitive.

Representative Mike Garcia, a Republican, won a special election to replace a young Democrat felled by a sex scandal, then shocked Democrats by winning re-election last year by 333 votes in a district that Mr. Biden won by 35,000. The commission, however, appears intent on lopping off Republican-heavy Simi Valley from Mr. Garcia’s district in north Los Angeles County, leaving him holding on by a thread to a considerably less conservative seat.

“It makes guys like me perk up and go, ‘OK, what was the rationale for dumping this?’” Mr. Garcia said of the commission’s decision. “When you go through all the questions that are, in my opinion, objective, the only thing you’re left with is a rationale that is political.”

Democrats are at risk, too. The commission has proposed eliminating the Los Angeles seat of Representative Lucille Roybal-Allard, who in 1992 became the first Mexican American woman elected to Congress. Representative Katie Porter, a hero of the national Democratic Party, appears likely to be left with a more Republican district in Orange County — a fate that could prompt her to run for the Senate instead, either by challenging Alex Padilla, the Democrat appointed to fill Vice President Kamala Harris’s seat, or waiting for Senator Dianne Feinstein, 88, to step aside.