"Somebody has to step up and lower the temperature," he said in a campaign launch video. "Somebody has to get things done."

Sununu's highly anticipated entry into the race could energize Republican hopes of reclaiming this seat, though he's not the only former US senator seeking the GOP nomination.

Sununu, 61, is up against Scott P. Brown, 66, who represented Massachusetts in the Senate, later served as ambassador to New Zealand, and now lives in Rye, N.H.

Brown launched his 2026 campaign over the summer as the first Republican candidate to enter the race. But polling conducted late last month by the University of New Hampshire Survey Center showed him trailing Sununu by a 23-point margin among likely GOP primary voters. A third Republican candidate, State Senator Daniel E. Innis, has already dropped out of the running and endorsed Sununu.

Sununu's family name represents a political dynasty in New Hampshire, where the list of past governors includes his younger brother Christopher T. Sununu and their father, John H. Sununu, who went on to serve as White House chief of staff for President George H. W. Bush.

Republicans in Washington, including President Trump, have nudged the Sununus to try and flip this Senate seat, but Chris Sununu -- who wrapped up his eight-year tenure as governor in January -- said in April he wouldn't enter the race.

Regardless of the cachet and connections that come with the Sununu family name, Brown has signaled he has no plans to clear the way for an uncontested Republican primary. In fact, this isn't his first time going toe-to-toe with a fellow former US senator in such a contest. He beat former senator Robert C. Smith in the 2014 primary, then lost to Shaheen in the general election.

That said, the UNH Survey Center polling suggests John E. Sununu may be better positioned than Brown for a general election showdown with Representative Christopher C. Pappas, the Democratic front-runner. With more than a year until the actual vote, the survey's hypothetical match-ups showed Pappas with a 6-point lead over Sununu and a 15-point lead over Brown.

While this race in New Hampshire may lean Democratic, analysts rank it among the most competitive contests on the horizon, as Republicans -- who currently hold 53 of the Senate's 100 seats -- hope to hang onto their majority in the midterm elections, which could mark a shift in the trajectory of Trump's second presidency.

There's also the open question of which GOP hopeful, if any, might win Trump's endorsement in the New Hampshire race. Brown has sought to position himself as an ardently pro-Trump candidate, and Sununu could face lingering blowback for criticizing Trump during the 2024 election cycle, when he wrote an op-ed calling Trump a loser.

Pappas, 45, is a well-connected politician with widespread name recognition in the state. The four-term congressman in New Hampshire's 1st Congressional District secured an endorsement from Shaheen, 78, before his Democratic opponents stepped forward.

Pappas, who has touted his status as one of the most bipartisan members of Congress, is up against Karishma Manzur and Jared Sullivan in the Democratic primary. Manzur is a progressive nonprofit leader and medical scientist from Exeter, N.H., and Sullivan is a current state representative in New Hampshire's North Country.

New Hampshire Democratic Party Chair Raymond Buckley released a statement Wednesday calling John E. Sununu a "corporate shill" who earned millions from private-sector business interests after leaving office.

"The only reason Sununu wants to go back to Washington now is to sell out New Hampshire to the same corporations and special interests that have lined his pockets for years," Buckley said.

John E. Sununu launches US Senate bid in New Hampshire, taking on Scott Brown for GOP nomination - The Boston Globe