Former Sen. John Sununu won the Republican primary for New Hampshire’s open Senate seat on Tuesday.

Endorsed by President Trump, Sununu will face Democratic Rep. Chris Pappas in November’s general election. Sununu easily defeated Scott Brown, the former U.S. senator from Massachusetts who was elected in 2010 to fill Ted Kennedy’s seat.

The race is for the Senate seat created when Senator Jeanne Shaheen, a Democrat, announced her retirement. Both Mr. Sununu and Mr. Brown had previously lost Senate races to Ms. Shaheen, who has served three terms. Ms. Shaheen beat Mr. Sununu in 2008 and Mr. Brown in 2014 after he moved from Massachusetts to New Hampshire.

Mr. Sununu, who served three terms in the U.S. House of Representatives and one term in the Senate, is the brother of former Governor Chris Sununu, who remains popular across the state, and the son of former Governor John H. Sununu. Mr. Brown expressed frustration that some voters confused John E. Sununu with his brother or father.

Sununu’s victory sets up a general election battle that could determine which party controls the U.S. Senate. Recent weeks have seen Democrats in New Hampshire warn that Sununu might defeat Pappas, the four-term congressman from the state’s swingy First Congressional District.

“As senator, I’ll have just one job: stand up for the people of New Hampshire every single day,” Sununu said in a statement. “We will keep listening, keep working, and stay laser-focused on the economy, jobs, and making life more affordable.”

Polls indicate an extremely tight race for the critical Senate seat. Trump endorsed Sununu in the primary against former Sen. Scott Brown of Massachusetts, who also served as an ambassador during Trump’s first term. With about one-sixth of the vote counted, Sununu held 70% support to Brown’s 25%.

First elected to Congress in 2018, Pappas has long been seen as the front-runner in the race to replace retiring Democrat Jeanne Shaheen. He launched an attack against Sununu in mid-August, stating: “In Congress, Sununu gave huge tax breaks to pharma companies and sided with the insurance industry, and then he went to work for them and made $35 million.”

Republicans have already begun targeting Pappas. One Nation, the nonprofit arm of the GOP super PAC Senate Leadership Fund, launched an ad accusing Pappas of “giving out tax breaks to New Yorkers for their million-dollar homes” when he supported a sweeping climate and social safety net measure in 2021.