A far-right Republican who backs Donald Trump’s election fraud lie and has vowed to decertify results in 2024 will be the GOP candidate for US Senate in New Hampshire.

Don Bolduc, a retired special forces general who has said he suffered from PTSD and a traumatic brain injury, edged out Chuck Morse, the state senate president, to face the incumbent Democrat, Maggie Hassan, in November.

Most if not all forecasters called the race for Bolduc before Morse conceded.

The primary was the last in a series that have seen Republicans select candidates aligned with Trump, causing some to fear damage to their chances of winning the Senate in November.

Bolduc, 61, has echoed Trump’s lie about election fraud in his defeat by Joe Biden. He has also questioned whether the FBI should be abolished following its search of Trump’s Florida estate, which turned up a cache of classified documents.

Though Bolduc has courted Trump, he has not won an endorsement. Trump did call Bolduc a “strong guy”.

Last October, Bolduc spoke to the New Yorker. He said he thought his “values and principles as an American, and the constitution, which I served for 33-plus years in the military, was safe with President Trump”, and that Trump’s appeal stemmed from the (notoriously reading-averse) former president’s reading and understanding of the constitution.

He also said “there was a tremendous amount of fraud” in 2020, adding: “I very much believe it and I think it exists, and I think it happens and it’s been happening for a long time in this country. When you try to steal the presidency, a lot of people are going to go, ‘OK, wait a minute. What the hell’s going on here?’”

On 6 January 2021, nine senators were among 147 Republicans who voted to object to results in key states, even after the Capitol was stormed by a pro-Trump mob, a riot now linked to nine deaths, including suicides among law enforcement.

Asked if he would “walk the walk” on certification in the Senate in 2024, Bolduc told the New Yorker: “Oh, absolutely … everybody I talk to believes that in me.”

Bolduc also said January 6 represented “a complete failure of the political system”, blaming “the speaker of the House, the Senate majority leader, the minority leader” and the vice-president, Mike Pence, who refused to reject electoral votes.

“They failed us,” Bolduc said, “and so, therefore, now they’re trying to politicize it, turn it into something that it’s not.”

He said Trump supporters should not have used violence and destroyed property, but “believed that their rights were violated. They believed that they lost their voice.”

Morse was endorsed by the popular Republican governor of New Hampshire, Chris Sununu, whose decision not to take on Hassan himself disappointed many Republicans.

Sununu called Bolduc a “conspiracy theorist”. Bolduc called Sununu a “Chinese communist sympathiser”. But the governor promised to “endorse whoever the nominee is, and support him, of course I will, no question”.

In a newsletter on Wednesday, J Miles Coleman of the University of Virginia Center for Politics said Sununu’s endorsement of Morse had almost been enough to defeat Bolduc. But he also pointed to Democratic efforts to boost the Trumpist Republican, mirroring controversial tactics in other states.

Coleman wrote: “Some Republicans complained that the Democratic-aligned Senate Majority PAC intervened against Morse – given the margin, this may have been the decisive factor, although Morse also got some help from Republican outside groups.”

One national Republican group spent at least $4.6m in support of Morse.

The UVA center rates the New Hampshire Senate contest as “leans Democratic”.

Linda Fowler, a political science professor at Dartmouth, told Reuters Morse would have stood a better chance of beating Hassan because he would have appealed to independents, the majority in New Hampshire.

“If Bolduc gets the nomination, the independents will go to Hassan,” Fowler said, speaking before the result was known. “If he doesn’t get the nomination, the independents will have a serious choice.”

Neil Levesque, director of the New Hampshire Institute of Politics, told the Associated Press Bolduc was the kind of candidate who would have struggled before Trump’s rise. Bolduc has never held elected office and had just $75,000 in cash on hand last week. But he was able to position himself as an ally of Trump.

“If it mirrors the former president, it’s been effective,” Levesque said.