President Donald Trump on Sunday endorsed former Sen. John E. Sununu in New Hampshire’s open Senate race, boosting a longtime critic over one of his former ambassadors, Scott Brown.

Trump hailed Sununu, who Republicans see as their best chance to flip the blue Senate seat, as an “America First Patriot” in a Truth Social post Sunday afternoon. And Trump said Sununu will “work tirelessly to advance our America First Agenda.”

“John E. Sununu has my Complete and Total Endorsement — HE WILL NOT LET YOU DOWN. MAKE AMERICA GREAT AGAIN — ELECT JOHN E. SUNUNU,” he posted.

Sununu, a moderate who has opposed Trump across his presidential runs, thanked him in a statement and quickly pivoted to talking about his priorities for New Hampshire.

“I want to thank the President for his support and thank the thousands of Granite Staters who are supporting me,” Sununu said. “This campaign has and always will be about standing up for New Hampshire — every single day.”

Trump’s endorsement further tips the scales in an already pitched GOP primary between Sununu and Brown, who represented Massachusetts in the Senate before moving to New Hampshire and running unsuccessfully for Senate there in 2014. He served as Trump’s ambassador to New Zealand and Samoa in his first term, and has been presenting himself as the more Trump-aligned candidate as he courts the MAGA base.

Brown vowed to fight on. And he took a veiled shot at Sununu, accusing him of not being sufficiently dedicated to the MAGA movement.

“I am running to ensure our America First agenda is led by someone who views this mission not as a career path, but as a continuation of a lifelong commitment to service,” Brown said in a post on X. “Let’s keep working.”

The two are competing to take on Democratic Rep. Chris Pappas for the seat being vacated by retiring Democratic Sen. Jeanne Shaheen. Pappas issued a simple response to Trump’s endorsement of Sununu: “I’m Chris Pappas, and I approve this message,” he wrote on X. His campaign manager, Rachel Pretti, said in a statement that Trump’s endorsement “confirms” that Sununu “will sell out Granite Staters to advance his political career.”

Trump’s support for Sununu once would have seemed unfathomable. The scion of a moderate New Hampshire Republican dynasty, Sununu served as a national co-chair of former Ohio Gov. John Kasich’s 2016 presidential campaign and joined his family in backing former U.N. Ambassador Nikki Haley for president against Trump in the 2024 GOP primary.

Ahead of New Hampshire’s 2024 presidential primary, Sununu penned an op-ed lambasting Trump as a “loser.” (Trump went on to win by 11 points). And he later derided Trump’s 2020 election conspiracies as “completely inappropriate.”

Republicans initially were bullish about flipping an open seat in purple New Hampshire that’s already changed hands between parties twice this century — Sununu defeated Shaheen to win the seat in 2002, then lost it to her in 2008 — and coalesced quickly behind the moderate Republican as their best option against Pappas. Sununu received instant backing from the GOP’s Senate campaign arm upon his launch last October and has wracked up endorsements from the majority of Republican senators. He’s also won support from Republican leaders in New Hampshire — all of which Trump noted in his Truth Social post Sunday.

Trump also initially supported Sununu’s younger brother, former Gov. Chris Sununu, running for the Senate seat. Chris Sununu, also a vocal Trump critic, declined to launch a bid, prompting GOP interest in his brother.

But some in Trump’s Granite State MAGA base quickly rejected his endorsement of Sununu, calling it a “slap in the face to grassroots supporters” long loyal to the president.

“The Sununu family openly mocked, degraded, and worked against the America First movement, the President himself, and the policies that energized New Hampshire voters,” a group of MAGA activists wrote on X. “We will continue and intensify our campaign opposition to the Sununu operation.”

Sununu holds a wide lead over Brown in polling of the GOP primary. The latest, a University of New Hampshire online survey of likely primary voters from mid-January, showed Sununu up 48 percent to 25 percent with 26 percent of likely voters undecided. But Pappas is ahead of both Republicans in hypothetical general-election matchups, leading Sununu by 5 percentage points and Brown by 10 percentage points in the UNH poll. The survey of 967 likely GOP primary voters had a margin of error of +/-3.2 percent.

Pappas also outraised both Republicans, bringing in $2.3 million last quarter and amassing a $3.2 million war chest heading into the year. Sununu hauled in $1.3 million and had $1.1 million in cash on hand in his primary campaign account while Brown raised $347,000 through his main account and had $907,000 in the bank.