Updated at 8:50 A.M. E.S.T.

WASHINGTON — Republican Kevin McCarthy was elected House speaker on a historic post-midnight 15th ballot early Saturday, overcoming holdouts from his own ranks and floor tensions that boiled over after a chaotic week that tested the new GOP majority’s ability to govern.

“My father always told me, it’s not how you start, it’s how you finish,” Mr. McCarthy told cheering fellow Republicans.

Eager to confront President Biden and the Democrats, he promised subpoenas and investigations. “Now the hard work begins,” the California Republican declared. He credited President Trump for standing with him and for making late calls “helping get those final votes.”

Republicans roared in celebration when his victory was announced, chanting “USA! USA!”

Finally elected, Mr. McCarthy took the oath of office, and the House was finally able to swear in newly elected lawmakers who had been waiting all week for the chamber to formally open and the 2023-24 session to begin.

After four days of grueling ballots, Mr. McCarthy flipped more than a dozen conservative holdouts to become supporters, including the chairman of the chamber’s Freedom Caucus.

Late on Friday he fell one vote short on the 14th ballot, and the chamber became raucous, unruly.

Mr. McCarthy strode to the back of the chamber to confront Representative Matt Gaetz, sitting with Representative Lauren Boebert and other holdouts. Fingers were pointed, words exchanged and violence apparently just averted.

At one point, Representative Mike Rogers of Alabama, shouting, approached Mr. Gaetz before another Republican, Mr. Richard Hudson, physically pulled him back.

“Stay civil,” someone shouted.

Order restored, the Republicans fell in line to give Mr. McCarthy the post he had fought so hard to gain, House speaker, second in the line of succession to the presidency.

CNN credited a phone call from Mr. Trump to Mr. Gaetz with the Florida congressman’s decision to drop his opposition to Mr. McCarthy’s bid.

The few remaining Republican holdouts began voting present, dropping the tally he needed. It was the end of a bitter standoff that had shown the strengths and fragility of American democracy.

The tally was 216-212 with Democrats voting for leader Hakeem Jeffries, and six Republican holdouts to McCarthy simply voting present.

The night’s stunning turn of events came after Mr. McCarthy agreed to many of the detractors’ demands — including the reinstatement of a longstanding House rule that would allow any single member to call a vote to oust him from office.

Even as Mr. McCarthy secured the votes he needs, he could emerge as a weakened speaker, having given away some powers and constantly under the threat of being booted by his detractors.

But he could also be emboldened as a survivor of one of the more brutal fights for the gavel in American history. Not since the Civil War era has a speaker’s vote dragged through so many rounds of voting.

Mr. Trump may have played a role in swaying some holdouts — calling into a meeting of Republican freshmen the night before, and calling other members ahead of voting. He had urged Republicans to wrap up their public dispute.

Electing a speaker is normally an easy task for a party that has just won majority control. But not this time: About 200 Republicans were stymied by 20 far-right colleagues who said Mr. McCarthy was not conservative enough.

The House adjourned Friday until late in the night, giving time for last-minute negotiations and allowing two absent Republican colleagues to return to Washington.

Newly elected Wesley Hunt of Texas arrived to vote for Mr. McCarthy — to applause, days after his wife had given birth — as did Ken Buck of Colorado.

The disorganized start to the new Congress pointed to difficulties ahead with Republicans now in control of the House, much the way that some past Republican speakers, including John Boehner, had trouble leading a rebellious right flank. 

The agreement McCarthy presented to the holdouts from the Freedom Caucus and others centers around rules changes they have been seeking for months. Those changes would shrink the power of the speaker’s office and give rank-and-file lawmakers more influence in drafting and passing legislation.

At the core of the emerging deal was the reinstatement of a House rule that would allow a single lawmaker to make a motion to “vacate the chair,” essentially calling a vote to oust the speaker. 

Mr. McCarthy had resisted allowing a return to the longstanding rule that Speaker Pelosi had done away with, because it had been held over the head of Mr. Boehner. But it appears Mr. McCarthy had no other choice.