As a co-founder of the conservative House Freedom Caucus, Representative Jim Jordan, Republican of Ohio, once antagonized his party’s leadership so mercilessly that former Speaker John A. Boehner, whom he helped chase from his position, branded him a “legislative terrorist.”

Less than a decade later, Mr. Jordan — a fast-talking Republican often seen sans jacket, known for his hard-line stances and aggressive tactics — is now one of two leading candidates to claim the very speakership whose occupants he once tormented.

Mr. Jordan’s journey from the fringe of Republican politics to its epicenter on Capitol Hill is a testament to how sharply his party has veered to the right in recent years, and how thoroughly it has adopted his pugilistic style.

Those forces played a pivotal role in the downfall of former Speaker Kevin McCarthy last week, though Mr. Jordan, once a thorn in his side, had since allied himself with Mr. McCarthy, a California Republican. Now, the same dynamics have placed Mr. Jordan in contention for the post that is second in line to the presidency, a notion that is mind-blowing to many establishment Republicans who have tracked his career.

“That notion that he could go from ‘legislative terrorist’ to speaker of the House is just insane,” said Mike Ricci, a former aide to both Mr. Boehner and Speaker Paul D. Ryan, Republican of Wisconsin. “Jordan is an outsider, but he’s very much done the work of an insider to get to this moment. Keeping that balance is what will determine whether he will win, and what kind of speaker he will be.”

The race between Mr. Jordan, a populist who questions federal law enforcement and America’s funding of overseas wars, and Representative Steve Scalise, a staunch conservative and the No. 2 House Republican from Louisiana, continued to heat up on Friday. Both men worked the phones relentlessly seeking support, including making calls with freshman lawmakers, the Congressional Western Caucus and the Main Street Caucus, a group of business-oriented Republicans.

On Friday, as they were vying for support, a bloc of Republicans were quietly requesting a change to party rules that would raise the vote threshold for nominating a candidate for speaker, which would make it more difficult for Mr. Scalise to prevail.

While Mr. Scalise is amassing dozens of commitments of support, so is Mr. Jordan, which could lead to a bitter and potentially prolonged battle when Republicans meet behind closed doors next week to choose their nominee — or spill into public disarray on the House floor.

Mr. Jordan’s rise in Congress to a position where he can credibly challenge Mr. Scalise, who has served in leadership for years, stems from a number of important alliances he has formed over the years. His strongest base of power is his colleagues in the House Freedom Caucus, many of whom consider him a mentor. He has built a solid relationship with Mr. McCarthy, for whom Mr. Jordan proved a reliable supporter and important validator on the right. And he has forged close ties with former President Donald J. Trump, perhaps his most important ally.

In a Republican House that has defined itself in large part by its determination to protect Mr. Trump and attack President Biden, Mr. Jordan has been a leader of both efforts. He leads a special subcommittee on the “weaponization of government” against conservatives. He has started investigations into federal and state prosecutors who indicted Mr. Trump, and he is a co-leader of the impeachment inquiry into Mr. Biden that Mr. McCarthy formally announced last month as he worked to appease the right and cling to his post.

Mr. Trump endorsed Mr. Jordan for the top House job early on Friday, ending speculation, however unrealistic, that the former president might seek the job himself. (A speaker is not required to be an elected lawmaker.)

“Congressman Jim Jordan has been a STAR long before making his very successful journey to Washington, D.C.,” Mr. Trump wrote in a post on his social media platform, Truth Social. “He will be a GREAT Speaker of the House, & has my Complete & Total Endorsement!”

Mr. Trump’s endorsement could help Mr. Jordan garner support from his other fellow House Republicans, among whom Mr. Trump is popular. But it is not expected to seal a victory.

Representative Warren Davidson, an Ohio Republican who is the whip of the House Freedom Caucus and a supporter of Mr. Jordan, said Mr. Trump’s endorsement was a “positive” for Mr. Jordan because “Trump is widely viewed as the leader of our party.”

But, he said, some more mainstream Republicans aren’t thrilled about aligning themselves with Mr. Trump.

“There are some folks in moderate districts that are like, ‘Well, that might actually complicate things for me,’” Mr. Davidson said.

Mr. Jordan helped undermine faith in the 2020 presidential election results as Mr. Trump spread the lie that the election had been stolen through widespread fraud. Mr. Jordan strategized with Mr. Trump about how to use Congress’s official count of electoral votes on Jan. 6, 2021, to reject the results, voting to object even after a mob of Mr. Trump’s supporters attacked the Capitol. His candidacy for speaker has drawn a stark warning from former Representative Liz Cheney of Wyoming, who was the No. 3 Republican and vice chair of the Jan. 6 committee, who said that if he prevailed, “there would no longer be any possible way to argue that a group of elected Republicans could be counted on to defend the Constitution.”

In a speech at the University of Minnesota this week, Ms. Cheney told the audience that “Jim Jordan was involved, was part of the conspiracy in which Donald Trump was engaged as he attempted to overturn the election.”

Mr. Jordan has defended his actions in challenging the results of the 2020 election, saying he had a “duty” to object given the way some states changed voting procedures during the coronavirus pandemic.

His quick rise in the Republican ranks was nearly derailed in 2018, when a sexual abuse scandal in Ohio State University’s athletics program came to light, leading to accusations that Mr. Jordan, who had been an assistant wrestling coach at the time, knew about the abuse and did nothing. Mr. Jordan has said that he was not aware of any wrongdoing.

On Capitol Hill, Mr. Jordan initially worked to build some relationships with Democrats early in his career. He and Representative Jamie Raskin, Democrat of Maryland, once teamed up on bipartisan legislation to protect press freedom. He counts former Representative Dennis Kucinich, a Democrat from Ohio who is now running Robert F. Kennedy Jr.’s presidential campaign, as a friend. Even as Mr. Jordan and Representative Elijah Cummings, the Maryland Democrat who died in 2019, sparred over investigations of Mr. Trump, the two men occasionally found common ground on other Oversight Committee issues.

But as Mr. Jordan formed an alliance with Mr. Trump and then became one of his most vocal defenders on Capitol Hill, his relationships with Democrats disintegrated. When Mr. Raskin introduced his press freedom bill this year, Mr. Jordan was no longer listed as a sponsor.

Representative Jim Banks, Republican of Indiana, said that Mr. Jordan’s true power lay in the love he commands from base voters, built up through years of defending Mr. Trump and advocating conservative policies on Fox News and in combative congressional hearings. Mr. Jordan is known to fly to districts around the country to help raise money for candidates who are aligned with the House Freedom Caucus — and even for Republicans who are not.

Mr. Banks suggested that Mr. Jordan’s credibility with the right would make it easier for the party to unify behind any spending deal he were to cut with Democrats and the White House should he become speaker. Such a deal would be a tall order. Mr. Jordan voted last week against a measure to avoid a government shutdown — an agreement with Democrats that ultimately drove Mr. McCarthy from the speakership.

“Jim Jordan is a trusted conservative; he’s well-respected by the base of the Republican Party,” Mr. Banks said. “So when we get to some of these tough spending fights and Speaker Jim Jordan is negotiating with the White House and the Senate, that’s going to help Republicans rally behind him and get to a place where they can vote for those deals.”

“This is a different Republican Party today than what it was a decade ago,” he added. “And the Republican Party today is a lot more like Jim Jordan. It’s more of a fighting Republican Party.”