Sununu, the popular GOP governor, had been outrunning Hassan in the early polls, despite now-President Joe Biden’s 7-point victory in New Hampshire last year. Sununu won reelection by 22 points on the same ballot.

The Republican governor’s decision is a major blow to Senate Republicans, who lost the majority earlier this year and are seeking to take it back in next year’s midterms. The GOP is defending two seats in states Biden won, and Sununu’s candidacy would have elevated New Hampshire to the top tier of offensive states for the party, joining Arizona, Georgia and Nevada.

Sununu told reporters he didn't give Republicans in D.C. a heads-up that he had decided against a campaign.

"I guess you'll have to let them know," he said in response to how Senate GOP leaders reacted to the news. "I haven't talked to them."

Democrats have been in campaign mode against Sununu for months. Hassan has already spent $2.7 million on TV ads for her reelection campaign, and Democratic groups have spent significant cash hitting Sununu on his support for new abortion restrictions.

"Senator Hassan won her last race by 1,017 votes, and we know that no matter who emerges as the Republican nominee this is going to be a hard-fought race," Hassan campaign manager Aaron Jacobs said in a statement.

The state Democratic Party released a lengthy memo last week claiming Sununu would enter the Senate race “from the weakest place in his political career” and linking him to Senate Minority Leader Mitch McConnell. The Republican governor has faced backlash from his own party for his handling of the Covid-19 pandemic, vaccine funding and Trump’s false claims about the 2020 election.

The Republican Senate primary includes Don Bolduc, a retired United States Army brigadier general who ran unsuccessfully for the state's other Senate seat in 2020, among other lesser-known candidates.

With Sununu out of the race, speculation shifted to former Sen. Kelly Ayotte (R-N.H.) as a potential Hassan challenger. Ayotte has been in touch with party leaders over the last year, but a source close to the former senator confirmed she will not run for Senate next year.

"Governor Sununu has done a tremendous job in office and I am so thankful he will continue his service to our state and continue to fight for all Granite Staters. As for Joe and I, we will continue to focus on our family, professional careers, and electing Republicans here at home," Ayotte said in a statement.

Former Sen. Scott Brown (R-Mass.), who moved to New Hampshire for an unsuccessful challenge to Democratic Sen. Jeanne Shaheen in 2014, will not run for Senate and will instead focus on his wife's congressional campaign, according to a source close to him. Gail Huff Brown, the former TV news reporter, is running against Rep. Chris Pappas (D-N.H.). Another Pappas challenger, 2020 GOP nominee Matt Mowers, is a potential Senate candidate, and local and national Republicans called him on Tuesday to gauge his interest in switching races.

New Hampshire has trended blue over the last two decades, though it remains a battleground. The last Republican to win New Hampshire’s electoral votes was George W. Bush in 2000, and Ayotte was the last Republican to win a statewide Senate contest there in 2010.

“We have lots of great candidates in New Hampshire, and we look forward to one of them beating Hassan next November,” National Republican Senatorial Committee spokesperson Chris Hartline said in a statement, which asserted that Hassan “is the least popular and most vulnerable incumbent” up next year.

Sununu outperformed former President Donald Trump by nearly 10 percentage points in the 2020 election. Thirty-seven percent of New Hampshire registered voters are unaffiliated with any party, according to the Secretary of State’s office, while 32 percent are Democrats and 31 percent are Republicans.

Sununu was first elected in 2016, beating Democrat Colin Van Ostern by 2 points, even as Trump narrowly lost the state. Hassan, the then-governor, won her Senate seat the same year, ousting Ayotte by approximately 1,000 votes.