WASHINGTON — Representative Devin Nunes, a California Republican who emerged as one of former President Donald J. Trump’s most loyal and pugnacious allies, announced on Monday that he would resign from Congress after 19 years to become the chief executive officer of Mr. Trump’s new media and technology company.
Mr. Nunes faced almost impossible odds in being re-elected to the Central Valley district that his family had farmed for three generations. A new map emerging from an independent citizens’ redistricting commission was almost certain to flip it from a district Mr. Trump won handily to one President Biden would have won.
But political analysts and politicians in the district had predicted that Mr. Nunes — who was the chairman, then the top Republican on the House Intelligence Committee during the Trump administration — would jump to a newly created, Republican-friendly district. That would have allowed him to continue his congressional career and assume the helm of the powerful Ways and Means Committee if Republicans took control of the House, as they are favored to do.
Mr. Nunes’s decision to take over Mr. Trump’s fledgling media enterprise instead of the influential House panel that writes tax and health care policy signals where he thinks power lies in the Republican Party and the conservative movement.
“Recently, I was presented with a new opportunity to fight for the most important issues I believe in,” Mr. Nunes wrote in a letter to his constituents, saying he would leave the House at the end of the year, well before the end of his term.
Mr. Trump, in a news release from the Trump Media & Technology Group, called his new chief executive “a fighter and a leader.”
“Devin understands that we must stop the liberal media and big tech from destroying the freedoms that make America great,” he said in a statement.
Mr. Nunes was once a trusted lieutenant of Speaker John A. Boehner, a Republican loyalist who clashed fiercely and publicly with the ardent conservatives in the House Freedom Caucus. Those Freedom Caucus rebels went on to become the vanguard of Mr. Trump’s takeover of the Republican Party, and Mr. Nunes followed suit.
From his perch on the Intelligence Committee, he ran interference for Mr. Trump against accusations that his 2016 campaign had collaborated with Russian intelligence. Mr. Nunes also organized a united Republican front opposing the first impeachment of the president for withholding military assistance to Ukraine to pressure its government to dig up dirt on Mr. Biden.
Mr. Nunes also waged public fights against the news media and his critics, suing The Washington Post, CNN, The Fresno Bee and Twitter over perceived slights, and going after Twitter in efforts to unmask the author of a mocking account ostensibly written by his cow. (His family runs a dairy operation.)
Phil Arballo, the Democrat who ran against Mr. Nunes last year and is again running for the seat in 2022, said he had expected his opponent to cut and run on the district, which according to an emerging map would include much more of urban Fresno while losing Republican strongholds to the east.
“What we know about Devin is he’s not going to work for anything,” Mr. Arballo said in an interview before the announcement on Monday.