Chris Christie wants to be very clear about something: The election of 2020 was not stolen.

“An election for president was held on November 3, 2020. Joe Biden won. Donald Trump did not,” Mr. Christie writes in his new book, “Republican Rescue: Saving the Party From Truth Deniers, Conspiracy Theorists, and the Dangerous Policies of Joe Biden.”

“That is the truth. Any claim to the contrary is untrue,” Mr. Christie says.

It is not a popular view in the Republican Party right now, as Mr. Trump has promoted his baseless claims of widespread election fraud for more than a year, and as many Republicans have either echoed those claims or averted their gaze.

But it’s a view that Mr. Christie has been repeating since Election Day, as he urges the G.O.P. — and Mr. Trump — to move on from looking backward.

“It’s not a book about him,” Mr. Christie said in a recent interview about the book, which will be released on Wednesday. “It’s a book about where we go from here and why it is important for us to let go of the past.”

Of Mr. Trump, Mr. Christie was blunt: “If he wants to be a positive force in the future, he’s got to let this other stuff go. If he doesn’t, I don’t think he can be.”

Mr. Christie pointed to the Virginia governor’s race and Glenn Youngkin, the Republican who won the state party convention without Mr. Trump’s endorsement and then kept him at bay during the general election. Mr. Youngkin ultimately defeated his Democratic opponent, Terry McAuliffe.

Mr. Christie said the Youngkin victory knocks down “this idea that if you don’t agree with Donald Trump on everything, and pledge unfettered fealty to him, then you can’t win because his voters quote unquote won’t come out to vote,” Mr. Christie said. “No candidate owns voters. They don’t.”

He described Mr. Trump’s conduct in the year since he left office — and the anxiety felt by lawmakers who worry about crossing him — in stark terms. “Donald Trump’s own conduct is meant to instill fear,” he said.

Mr. Christie is a former governor of New Jersey, a former presidential candidate and a possible future one. He was one of Mr. Trump’s earliest supporters in 2016 after he ended his own national candidacy, was a potential vice-presidential candidate, led Mr. Trump’s transition effort until he was fired from that role and helped lead Mr. Trump’s opioids commission.

He was with Mr. Trump throughout a tumultuous presidency, a fact that Mr. Christie’s critics say makes his critiques too late to be meaningful. Mr. Christie argues that his support for Mr. Trump, and their 15-year friendship before that, makes him a credible critic.

“I think it was really important for people to understand why I did support the president for so long,” Mr. Christie said. “And the reason was, because I generally agreed with the policies that he was pursuing.” When they would argue over the years, he added, “it was rarely over policy.”

The arguments were generally over how things were handled, Mr. Christie added, citing Mr. Trump’s throwing of “bouquets” at President Xi Jinping of China as an example. Being generous with Mr. Xi when the Chinese government was withholding information about the coronavirus was “unacceptable,” Mr. Christie said.

Mr. Christie does not blame Mr. Trump’s speech on Jan. 6 for the violence that followed at the Capitol by his supporters. He said instead that it was the months of Mr. Trump’s false claims that the election was stolen from him that instilled anger in those who believed him.

The responsibility for what happened “was months long in coming,” he said. “As a leader, you need to know that there are consequences to the words you use. And that those consequences at times can be stuff that you may not even be able to anticipate. I don’t believe he anticipated that people would cause violence up on Capitol Hill. But I don’t think he thought about it, either.”

Mr. Christie began road-testing his themes in a speech at the Reagan presidential library in September, during which he didn’t name Mr. Trump. When he spoke again at the Republican Jewish Coalition conference in Nevada last weekend, Mr. Trump took notice, and delivered a broadside that his aides intended as a warning shot.

Mr. Christie “was just absolutely massacred by his statements that Republicans have to move on from the past, meaning the 2020 Election Fraud,” Mr. Trump said in a statement that also attacked Mr. Christie for a low approval rating, which Mr. Trump mischaracterized by half.

Mr. Christie said that Mr. Trump should focus less on “personal vendetta,” and added, “I just think if he wants to have that kind of conversation about me then I’m going to point out that I got 60 percent of the vote in a blue state with 51 percent of the Hispanic vote.”

Mr. Christie said he would not make a decision about running for president in 2024 until after the midterm elections in 2022. He said that Mr. Trump would not factor into his thinking and that he would not rule out supporting the former president if he saw no path for himself.

Throughout the book, Mr. Christie places Mr. Trump in the historical context of a political strain in the country that is centuries old. The QAnon conspiracy theorists of the last several years are in many ways the descendants of John Birch Society members, Mr. Christie writes, and he contrasts how Ronald Reagan handled extremist voices in his party with how Mr. Trump has.

He faults Mr. Trump for spreading “the birther campaign” about former President Barack Obama’s birthplace in 2011.

“He truly showed everyone how a lie like that can be exploited,” Mr. Christie said, taking note of other Republicans who encouraged questions about where the first Black president was born.

And Mr. Christie writes that he knows Mr. Trump was furious after he was laughed at during the White House Correspondents Dinner in 2011, when Mr. Obama roasted him over his birther crusade. Mr. Trump later boasted that he was unbothered, but Mr. Christie said he spoke with Mr. Trump about it. “Just beside himself with fury,” Mr. Christie writes.

Mr. Christie also describes some of the debate prep sessions that he led for Mr. Trump before he took the stage with President Joseph R. Biden Jr. last year. In one session, Mr. Trump turned to Mr. Christie and began to excoriate him for recommending Christopher Wray for F.B.I. director.

“He’s doing an awful job, and he’s your pick. He was pick,” Mr. Trump told Mr. Christie in front of a half-dozen other Trump aides.

“Hold on a second,” Mr. Christie replied, praising Mr. Wray. “He wasn’t my pick. He was pick. He was my recommendation. I’m not the president. I don’t get to pick.”

Mr. Christie reveals how worried he and others were for his survival when he became infected with the coronavirus after being at the White House around the same time that Mr. Trump and several other aides contracted Covid-19. Mr. Christie writes that his priest arrived in the hospital and rubbed oils on his forehead in the sign of the cross, praying over him.

He got a call from a hospitalized Mr. Trump, who had one main concern: “Are you gonna say you got it from me?” Mr. Trump asked him.

Mr. Christie is unsparing in the book about Mr. Biden, whose policies he says he cannot align himself with. In the interview, he faulted the president for running as one kind of politician but governing as another, citing the aftermath of the withdrawal from Afghanistan as an example.

“If they had known how he was going to govern,” Mr. Christie said of voters, Mr. Biden may not have won.