FRESNO, Calif. — For nearly three years, Phil Arballo has been running for Congress against Representative Devin Nunes, the Republican that Democrats across the country have loved to loathe, raising money by the truckful and compiling an email outreach list that is all the more impressive considering his lack of political experience.

On Monday, Mr. Nunes announced he would resign from Congress at year’s end to lead former President Donald J. Trump’s media and technology company, continuing an unswerving fealty to Mr. Trump that had turned him into a national figure of admiration on the right and contempt on the left.

Mr. Nunes was prodded toward that decision in large part by the nonpartisan California Citizens Redistricting Commission, which this week is putting the finishing touches on new boundaries that are likely to transform the district he has represented for 19 years from a dusty, rural swath that voted for Mr. Trump in 2020 by 5 percentage points into one centered here in Fresno, the fifth-largest city in California, which Joseph R. Biden Jr. would have carried handily.

Mr. Arballo, who lost to Mr. Nunes last year and had been hoping to challenge him again, realizes he will have a different opponent.

“It’s going to be fun, though,” Mr. Arballo said, speaking from his spare campaign headquarters in a nondescript office park here. “And what we can do is also wash away the gerrymandering that’s going to be happening all over the country.”

Legislatures from Nevada to Georgia are drafting new House district lines under the required reapportionment that occurs every 10 years. Most of them are seeking to protect incumbency and maintain a partisan edge by eliminating competitive seats, a process that Republicans in particular have exploited to gain a heavy early advantage in their push to wrest control of the House next year. The Justice Department filed suit on Monday against a Texas map gerrymandered by the Republican-led legislature that would make that state redder, potentially leaving only a single district in play.

But in California, the map will stand in stark contrast to most of the country, scrambling the fortunes of lawmakers in both parties and creating the broadest — perhaps the only — true battlefield for 2022. Lawmakers should see the full plan by Friday, and the commission will send it to the secretary of state by Dec. 27.

Legislatures in nine other states, working off the 2020 census, have completed new maps of 116 House districts. In only 10 of those would the candidate who won 2020 have prevailed by 7 percentage points or less, according to the Princeton Gerrymandering Project; that is half the number of competitive districts that existed in 2018 and 2020.

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.

California’s 10th Congressional District, currently represented by Representative Josh Harder, a young, up-and-coming Democrat, will become heavily Republican, most likely sending Mr. Harder in search of a new district. (It was the expected destination of Mr. Nunes.) That could cost the quiet backbench Democrat Jerry McNerney, who might find himself a sacrificial lamb.

The former governor who set the process in motion, Arnold Schwarzenegger, is watching the free-for-all with glee. When he took office in 2003, he had never thought of redistricting reform, he said in an interview last week. But what he found was a system he called “wacky,” in which Democrats and Republicans came together every 10 years to redraw the lines of State Assembly districts, State Senate seats and U.S. House seats to preserve the status quo — politicians picking their voters, not the other way around.

“It was worse than the Politburo,” said Mr. Schwarzenegger, a Republican who came to office after a recall election. “The Constitution says, ‘We the people,’ not ‘We the politicians.’”

From 2002 to 2010, one California congressional district changed party hands. Since 2012, when the first map of Mr. Schwarzenegger’s redistricting commission went into effect, 16 seats have flipped. He called it “without doubt” one of his proudest achievements.

The commission includes five Republicans, five Democrats and four members not affiliated with a party, selected from citizen applicants. Commissioner J. Ray Kennedy, a Democrat, said the panel must create districts of equal population that are contiguous and compact, and to the extent practicable, keep counties, cities, neighborhoods and “communities of interest” together.

A person should be able to walk from any part of a district to another without crossing into a different one, though bulges and loops do form to comply with the Voting Rights Act’s requirement that minority voters get representation. Competitiveness is not a criterion, but it is a byproduct.

Compliance with the Voting Rights Act could create the first two Latino districts in the Central Valley, to the detriment of two Republicans: Mr. Nunes and Representative David Valadao, who will square off next year with Rudy Salas, a member of the State Assembly and a prime Democratic recruit. The district remains highly competitive but will slightly shift from Fresno and into Mr. Salas’s stronghold of Bakersfield.

“The way that the commission is looking at this independently, it’s actually shifting the district toward my home base, Kern County, which is my media market, where they’ve known me for at least 12-plus years since my time at City Council, and now with the State Assembly,” Mr. Salas said on Tuesday. “So I feel very confident.”

The contrast between California and the rest of the country is stark.

In Georgia, Republican legislators collapsed two competitive districts won narrowly by Democrats into one heavily Democratic district in suburban Atlanta. The state will have no competitive districts next year.

In Utah, lawmakers divided Democratic Salt Lake City into four outlying Republican districts, eliminating the state’s only swing seat.

For the past two House election years, three out of Iowa’s four House seats have been highly competitive. Next year, none will be.

“The reduction in competition is striking,” said Samuel S.H. Wang, the director of the Princeton project.

Texas was in 2018 and 2020 what California will be in 2022: the biggest battlefield of all, with eight competitive seats. Next year, the Lone Star State will have a lone seat — maybe two — in play, out of 38. And because the Supreme Court, in a 5-4 decision, ruled in 2019 that federal courts cannot hear challenges to partisan gerrymandering, citizens’ only recourse may be ballot initiatives like the one Mr. Schwarzenegger set in motion.

No one would say California has found the perfect system. Mr. Arballo said the commissioners did not appear to be familiar with the Central Valley. Supporters of Mr. Garcia lobbied the commissioners to keep Simi Valley and Santa Clarita together as “communities of interest.” But the effort was dismissed by the commissioners as a partisan play, said Paul Mitchell, a Democratic consultant and the state’s demographer.

Without Simi Valley, fresh-faced Democrats like John Quaye Quartey can focus their attention on diverse areas around Santa Clarita, near Los Angeles, and more working-class terrain around Palmdale and Lancaster.

Mr. Quartey has a dream biography for the Democratic Party: The son of a Ghanaian immigrant, he went to the Naval Academy to play football and had a 20-year career as a Naval intelligence officer and military diplomat.

With the discipline of the military veteran he is, Mr. Quartey refused to guess at how a changed map would affect his campaign.

“However the maps are drawn, it’s going to be competitive,” he said outside a coffee shop in Santa Clarita.

His Republican opponent, Mr. Garcia, is not about to give up his beef with the mapmakers. He questioned how citizens were supposed to have any input if their objections were rejected as politically motivated.

“Some people are starting to question whether or not these guys are doing this with truly objective intent,” he said.

Representative Brad Sherman, a Democrat who had to defeat another Democrat in 2012, after the commission’s first map collapsed their districts together, had a different complaint: If Republican-controlled states like Florida, Texas and Ohio are skewing their maps for partisan advantage, he argued, the largest Democratic state needs to be able to counter — and it cannot.

“When you have a system that says we’re going to have purity in California and skulduggery in Texas, you end up with an unrepresentative chamber,” Mr. Sherman said. “We want to live in a system where neither party gets screwed. But worst of all is a system where only one party gets screwed.”