President Trump on Friday ordered the FBI to reopen the investigation of Supreme Court nominee Brett M. Kavanaugh’s background, a stunning turnaround in an emotional battle over sexual assault allegations that has shaken the Senate and reverberated across the country.

The dramatic developments capped an extraordinary day on Capitol Hill, which began with a sense of momentum for Kavanaugh but then sharply changed when one of Trump’s fiercest Republican critics, Sen. Jeff Flake (Ariz.), who at first endorsed the nominee, emerged from a private meeting with Democrats to call for a renewed inquiry into misconduct allegations.

Flake’s move puts in doubt the fate of Kavanaugh, who has in recent days drawn strong support from Trump and other top Republicans, but now faces another week of scrutiny and must watch as senators in both parties endure mounting pressure from their ­respective bases to either rally to his side or block his confirmation.

Conservatives have demanded that Republicans confirm Trump’s second Supreme Court pick, a judge who could shift the high court to the right for a generation, or face political consequences in midterm elections in six weeks with the control of Congress at stake.

The delay once again thrusts the FBI, an increasing target of Trump’s ire, into the center of a politically charged controversy.

Flake, who at one point Friday was confronted by two women who tearfully and angrily urged him to consider the pain of sexual assault survivors, was soon joined by Sens. Lisa Murkowski (R-Alaska) and Susan Collins (R-Maine), and hailed by Democrats for forcing a one-week delay in the nomination process to allow for the federal inquiry.

“There’s lingering doubt out there among a lot of people that we haven’t taken every measure that we should to make sure that these allegations are dealt with,” Flake told reporters Friday after the Senate Judiciary Committee said it requested the White House call for an FBI probe. “That’s what this effort is about.”

The request for a supplemental FBI background check will almost certainly delay Kavanaugh’s confirmation vote beyond Tuesday, the tentative date eyed by Senate GOP leaders. But Trump and Senate Majority Leader Mitch McConnell (R-Ky.) had no choice but to placate the small core of undecided senators whom Trump has little power to influence. 

Late Friday, by voice vote, the Senate took an initial step to move ahead on the nomination. Barring no major revelations from the FBI, the Senate could vote on confirming Kavanaugh next weekend, days after the start of the high court’s session.

The FBI has already cleared Kavanaugh through six background checks for his previous public-service jobs, a point the nominee underscored in a statement Friday while adding, “I’ve done everything they have requested and will continue to cooperate.”

Trump said the investigation “must be limited in scope and completed in less than one week.” The Judiciary Committee said the probe would cover “current credible allegations,” although committee spokesmen did not respond to a request to elaborate on what those accusations are. 

The rapid-fire events Friday came a day after dramatic, searing testimony from Christine Blasey Ford, who has accused Kavanaugh of sexual assault when both were in high school, and from the nominee, who denied the allegation in an emotional presentation.

Attorneys for Ford praised the decision to reopen the probe and added, “No artificial limits as to time or scope should be imposed on this investigation.”

The announcement followed a vote along party lines by the Judiciary Committee to advance Kavanaugh’s nomination — an 11-to-10 roll call that was left significantly in doubt amid a contentious yet odd debate as senators dashed into an anteroom behind the hearing space. 

The committee fight already was tense, as several Democrats — including Sens. Richard Blumenthal (Conn.), Mazie Hirono (Hawaii), Kamala D. Harris (Calif.) and Sheldon Whitehouse (R.I.) — walked out in protest. Flake often appeared conflicted and pained as senators engaged in a contentious debate over the nomination.

After Sen. Christopher A. Coons (D-Del.) implored Republicans to join Democrats in calling for an FBI probe, Flake walked over and gestured to Coons to join him in the anteroom. The two close friends, as well as Sen. Amy Klobuchar (D-Minn.), left — prompting immediate speculation about whether they were working on a deal to delay the vote.

The curiosity inside the Dirksen Senate Office Building hearing room grew as more and more senators shuffled in and out of the back room for the discussions, which Collins called into via phone. Besieged by colleagues, Flake and Coons even sought privacy for their conversations in one of the Senate’s old-fashioned phone booths.

The senators eventually returned, and Flake announced that he would still vote to advance Kavanaugh’s nomination, on the condition that a final vote would be delayed no more than a week to allow for an investigation. 

Shortly afterward, Murkowski, Collins and Sen. Joe Manchin III (D-W.Va.) said they supported his call for a limited FBI probe. Manchin said in a statement that “the American people have been pulled apart by this entire spectacle.”

“What I’m encouraged by is that in an anteroom, back-hall conversation — first with Sen. Flake and then with Sen. [Dianne] Feinstein and then with many other senators of both parties, there was broad agreement that this committee has been too divided and too partisan with this process,” Coons said.

Senate Republican leaders have yet to lock down the minimum 50 votes to confirm Kavanaugh. Sen. Doug Jones (D-Ala.) announced late Thursday that he would oppose the nominee, and on Friday, Sen. Joe Donnelly (Ind.) — one of just three Democratic senators who supported now-Justice Neil M. Gorsuch in April 2017 — said he would reject Kavanaugh as well. 

Collins and Murkowski have not declared a position on Kavanaugh’s nomination; neither has Manchin nor Sen. Heidi Heitkamp (D-N.D.). 

Trump told advisers in meetings Thursday and Friday that prolonging the vote doesn’t help Kavanaugh but that if an FBI check came up clean, he could use that as a cudgel to get more votes. But the biggest concern in the White House is that with more time, more women, more accusations and more stories could emerge. 

“It’s not the FBI investigation that sparks the fear like it is just another week of this,” one White House official said. “How much longer is this sustainable?”

On Thursday, Trump was elated with Kavanaugh’s fiery testimony, in which he defended his character while furiously denying the allegations. Trump was “fired up” in a phone call with McConnell about Kavanaugh’s ultimate confirmation, according to two people familiar with the call, and said senators needed to take the vote.

It was a reversal from an earlier call with McConnell in which Trump was upset as Ford delivered her emotional testimony, according to the two people, who spoke on the condition of anonymity to discuss private conversations. 

McConnell, as well as White House counsel Donald McGahn, has urged Trump to hold his fire and let the confirmation battle play out, advisers to both men said. Both Trump and McGahn had told others that they did not want an FBI investigation, two senior officials said — but that it was inevitable once it became clear that Kavanaugh could not be confirmed without one. 

McGahn and informal Trump judicial adviser Leonard Leo — rather than the president — have been calling senators. McGahn has told others that he has good relationships with Collins and Murkowski. 

Trump is telling advisers that he wants to stick by Kavanaugh. But he also says that “Republicans have been played like a fool,” according to a senior White House official. Still, one official involved in the confirmation process said that while Trump has expressed some frustration with the timing, he generally believes that McGahn and McConnell have the Supreme Court and other judicial nomination fights figured out.

Mark Judge, a friend and high school classmate of Kavanaugh’s, is likely to be a prominent figure in any inquiry by the FBI. Ford claims he was present when Kavanaugh allegedly attacked her. Another Kavanaugh accuser alleges that Judge and Kavanaugh sought on multiple occasions in high school to drug inebriated girls for nonconsensual sex with multiple boys — an accusation Kavanaugh has strongly denied.

“If the FBI or any law enforcement agency requests Mr. Judge’s cooperation, he will answer any and all questions posed to him,” Judge’s lawyer Barbara Van Gelder said.

Judge met with his lawyer Friday morning in Washington, after returning from Bethany Beach, Del. The Washington Post found him there Monday, where his lawyer said he had fled to try to avoid an avalanche of media requests and criticism. 

Judge told the Senate Judiciary Committee on Friday that he either does not recall or flatly rejects the allegations about his and Kavanaugh’s behavior in high school.

“All I can say is that next week, this will be over,” Sen. Lindsey O. Graham (R-S.C.) said. “I really believe, I believe he will be on the court.”

Carol D. Leonnig, Sean Sullivan, Mike DeBonis, Paul Kane, Robert Barnes and Elise Viebeck contributed to this report.