Republican Jeff Flake, the senator in the middle of an acrimonious fight over Supreme Court nominee Brett M. Kavanaugh, warned Monday that the FBI must thoroughly investigate allegations against the federal judge, as he made a long-shot plea to use the moment to bridge the divides that have deepened under President Trump.

“We’re wanting to make sure that is a fulsome investigation — that it’s not limited as some worry that it might be,” Flake told reporters here.

Decrying tribalism, partisanship and “the politics of vengeance” in a speech that served as an unmistakable shot at Trump, the Arizonan signaled he intends to use the polarizing court fight to amplify his long-standing calls for more civility and cooperation in Washington and across America.

“Tribalism is ruining us. It is tearing our country apart. It is no way for sane adults to act,” said Flake, a possible 2020 presidential candidate.

But the near impossibility of his task came sharply into focus in this early primary state as he faced dueling pressure from both sides of the Kavanaugh divide after forcing the Senate to delay a vote so the FBI could reopen its background check of the federal appeals court judge.

Flake remains a pivotal swing vote with the potential to sink Kavanaugh’s nomination, making him the target of liberal demands to keep him off the court, as evidenced by the anti-Kavanaugh protesters he faced here and in Boston on Monday. He is also the focus of conservative insistence that Kavanaugh be swiftly confirmed.

Sen. Jeff Flake (R-Ariz.) spoke at the New Hampshire Institute of Politics at Saint Anselm College in Manchester, N.H. (Charles Krupa/AP)

As he awaits the outcome of the FBI probe, which will determine his decision, he is poised to be a hero to one side and a villain to the other. “I sometimes feel like a man temporarily without a party,” Flake said.

But he is not without an audience. Flake is trying to turn the Kavanaugh standoff into a teachable moment that he hopes will help usher in a new political era, one that will produce greater compromise and expand the appeal of a Republican Party he says has put its long-term survival at risk by appealing to a narrow swath of Americans.

That pitch has become increasingly consequential as Flake, one of Trump’s most vocal GOP critics, prepares to retire from the Senate. His presence in this first-in-the-nation primary state has stoked speculation that he will run against Trump in 2020, a possibility he did not rule out.

It has become a career-defining moment for the mild-mannered lawmaker, who is at the epicenter of the collision between the #MeToo movement and the Trump presidency as he pursues pragmatism in a Senate riven by strident partisanship.

In his speech at the New Hampshire Institute of Politics at Saint Anselm College, Flake emphasized the themes that he has returned to repeatedly during Trump’s presidency, preaching civility and comity over rancor and recriminations.

“We Republicans have given in to the terrible tribal impulse that first mistakes our opponents for our enemies,” Flake said. “And then we become seized with the conviction that we must destroy that enemy.”

Earlier in the day, Flake spoke in Boston at a summit hosted by Forbes. His appearance had to be moved for safety reasons, organizers said. “The Republican Party is the president’s party right now,” he said there. “But it won’t always be. And it can’t be if we’re going to be a major political force in the future.”

Liberal protesters including congressional hopefuls Ayanna Pressley and Alexandria Ocasio-Cortez, favorites of the left, gathered at a nearby rally, highlighting the urgency with which they are approaching this week.

“As soon as I found out Jeff Flake would be here in Boston, I said, ‘Where is the rally?’ ” said Olympic figure skater Adam Rippon. His message to Flake: “Please listen to us: Vote no on Brett Kavanaugh.”

Democrats have voiced concerns that the scope of the FBI probe will be limited. Flake urged the FBI to conduct a “real investigation” and said he was in touch with the White House counsel’s office and his colleagues to ensure that happens.

“It does no good to have an investigation that just gives us more cover,” Flake said. “We actually need to find out what we can find out. And we have to realize that we may not be able to find out everything that happened.”

Flake said he is “waiting for the additional information that will come from the supplemental FBI investigation” to inform his decision on how to vote on Kavanaugh. He said the thrust of the probe is to see whether there is corroboration for the account Christine Blasey Ford delivered under oath at a Senate hearing last week.

Ford said Kavanaugh sexually assaulted her when they were in high school. Kavanaugh has denied the claims. Flake wants the FBI investigation to conclude this week and allow the Senate to move to a vote.

If Kavanaugh lied, Flake said, that should disqualify him. Senate Republicans hold a 51-49 advantage in the Senate, and Flake struck a deal with a handful of moderates from both parties to postpone the vote until the probe is conducted. That same coalition could defeat Kavanaugh’s nomination if they all decide to vote no.

If Kavanaugh’s nomination fails, it could further damage Flake’s troubled standing with many conservatives, including here in New Hampshire.

“If Judge Kavanaugh goes down, Jeff Flake should not ever bother coming back to the state,” said David Carney, a veteran GOP strategist. “He’s persona non grata here.”

GOP organizers here said they have not seen signs that Flake is trying to lay the groundwork for a presidential bid. But they acknowledged an appetite among some Republicans in the state for a new GOP standard-bearer.

“Challenges to incumbents don’t build over the course of years — they break late, and they happen all at once,” said Fergus Cullen, a former state GOP chairman who opposes Trump. “It’s not like an open-seat race that builds through Rotary Clubs and house parties and grass-roots activism.”

For now, there are few others so openly willing to challenge Trump the way Flake has done in speeches, tweets and a book he wrote last year. Flake wears his resistance as a badge of honor.

“I had failed my tribe. I’ve been failing my tribe for some time now,” he said.

His biggest test will be his Kavanaugh vote. While he forged some compromise to buy time for an FBI probe, the national uproar on both sides has shown no sides of easing.

Flake said in an interview with CBS’s “60 Minutes” that he would not have stood up to his party leaders like he has on Kavanaugh if he were running for reelection.

While he is freed from the constraints of a campaign for the moment, his vote is shaping up as a defining feature of his legacy. He is hoping he will also be remembered for what he did leading up to that time.

“If the past week has taught me anything, it’s that this country is hungry for us to work together again on their behalf,” he said.