Former President Donald J. Trump is leading an all-out war against Representative Liz Cheney of Wyoming because of her perceived lack of loyalty: After voting to impeach him, she became the voice of Republican opposition to his attempt to overturn the 2020 election.

But his choice to replace her, Harriet Hageman, was not always a loyal soldier herself. She was part of the final Republican resistance to his ascent in 2016, backing doomed procedural measures at the party’s national convention aimed at stripping him of the presidential nomination he had clinched two months earlier.

Ms. Hageman worked with fellow supporters of Senator Ted Cruz of Texas in a failed effort to force a vote on the convention floor between Mr. Trump and Mr. Cruz, regardless of the results of the primaries and caucuses held across America. Calling Mr. Trump “the weakest candidate,” Ms. Hageman attributed his rise to Democrats who she claimed had voted in Republican primaries.

She condemned Mr. Trump as a bigoted candidate who would repel voters Republicans needed to win a national election, warning that the G.O.P. would be saddled with “somebody who is racist and xenophobic.”

Ms. Hageman’s yearslong journey from Never-Trumpism to declaring him the best president of her lifetime is one of the most striking illustrations yet of the political elasticity demonstrated both by ambitious Republicans in the Trump era and by the former president himself, who has relentlessly asserted his dominance over leaders of his party.

Ms. Hageman is hardly the only Republican to vigorously oppose Mr. Trump and later back him when it proved politically advantageous. Mr. Cruz and Senator Lindsey Graham of South Carolina, along with Kenneth T. Cuccinelli II, who led the 2016 rebellion at the convention, all became enthusiastic Trump supporters.

None of them, however, have quite achieved Ms. Hageman’s remarkable political transformation, which has not been previously reported. Five years ago, she was a passionate opponent of Mr. Trump who tried to stop him outside the normal electoral process; now, she is his champion in the Republican Party’s marquee showdown over fealty to the former president.

Ms. Cheney’s vocal opposition to Mr. Trump has turned what might otherwise be a sleepy contest for a safely Republican Wyoming congressional seat into a high-profile test case of the former president’s dominance over the party. His obsession with removing Ms. Cheney from office — he has derided her in at least 16 statements since March, including one on Thursday that contained a doctored photo combining her hair, body and eyeglasses with former President George W. Bush’s face — has overshadowed nearly all of his other political efforts, aside from vying to overturn the results of the 2020 election.

“It’s going to be the most important House race in the country in 2022,” Ms. Cheney said during a “60 Minutes” interview broadcast on Sunday. “It will be one where people do have the opportunity to say, ‘We want to stand for the Constitution.’”

For Ms. Hageman, joining forces with Mr. Trump to attack an old ally — the two Wyoming women were once so close that Ms. Hageman served as an adviser to Ms. Cheney’s short-lived 2014 Senate campaign — presents an opportunity to accomplish something she has been unable to do without him: win a statewide race in Wyoming.

Ms. Hageman has never spoken publicly about her effort to block Mr. Trump from the 2016 nomination. In a statement to The New York Times, she drew a tenuous connection between her actions and Ms. Cheney.

“I heard and believed the lies the Democrats and Liz Cheney’s friends in the media were telling at the time, but that is ancient history as I quickly realized that their allegations against President Trump were untrue,” Ms. Hageman said. “He was the greatest president of my lifetime, and I am proud to have been able to renominate him in 2020. And I’m proud to strongly support him today.”

Ms. Cheney, who declined to comment or be interviewed for this article, supported Mr. Trump’s 2016 campaign. She endorsed him that May and, in October, issued a statement reiterating her support after the release of the “Access Hollywood” recording in which Mr. Trump bragged about groping women.

The daughter of a longtime Wyoming state legislator, Ms. Hageman, 58, built her career as a water and natural resources lawyer fighting environmentalists and government regulations. She became known in Wyoming for her successful challenge of a Clinton-era prohibition on road construction on millions of acres of U.S. Forest Service land. In 2009, a headline in an environmentalist magazine called her “The Wicked Witch of the West.”

In 2016, Ms. Hageman went to the Republican convention in Cleveland as a Cruz delegate after the Texas senator won Wyoming with 66 percent of the vote and 23 of 25 delegates at the state’s county conventions that March.

She had been appointed by the Wyoming delegation to the national convention’s powerful Rules Committee. The big question facing the committee’s members that year was how much say delegates should have in choosing the party’s nominee.

Leading up to the convention, Ms. Hageman joined a small group of Republicans who organized a last-ditch effort to “unbind” delegates. They hoped to insert a conscience provision freeing delegates to vote for whomever they wanted regardless of the results of state primaries and caucuses, a move concocted by supporters of Mr. Cruz to instigate a convention floor fight.

That summer, Ms. Hageman was a regular participant in conference calls plotting the last-gasp opposition to Mr. Trump, long after he had won enough delegates to clinch the nomination. She and other delegates, many of them social conservatives from the West loyal to Mr. Cruz, argued that Mr. Trump was a cancer on the party, chosen by liberal voters in Democratic states to undermine Republicans nationwide.

The Republican National Committee, working with the Trump campaign, did all it could to squash the rebellion.

“To vote to free the delegates at that time was considered a capital offense by the Trump campaign,” said Steve Duprey, then a Republican National Committee member from New Hampshire who was on the Rules Committee. “It was clearly an attempt to deny him the nomination, which he had won fair and square.”

Reince Priebus, then the chairman of the R.N.C., held long meetings with Mr. Cuccinelli and Rules Committee members who were seeking to unbind delegates. Ms. Hageman, along with Senator Mike Lee of Utah, who was at the time the highest-profile Rules Committee member involved in the stop-Trump movement, was among the attendees.

It soon became clear the Trump team had peeled away enough support from Mr. Cuccinelli that the vote would not be close. Mr. Trump’s allies forced a vote that would affirmatively declare delegates to be bound by the results of their state’s nominating contest.

When it was time to vote, 87 stood in favor of binding delegates.

Only 12, including Ms. Hageman and Mr. Lee, voted in opposition, far short of the 28 needed to put the question of unbinding delegates to a vote of the full convention, which would have been a potentially embarrassing spectacle for Mr. Trump. Though the fight was over, Ms. Hageman participated in meetings over the next few days in which Cruz delegates discussed whether they had any remaining options to stop Mr. Trump.

Mr. Trump, who endorsed Ms. Hageman this month, is aware of her support for Mr. Cruz in 2016 and, during his interview with her this summer before he made his decision, briefly touched on her role in the effort to stop Mr. Trump from claiming the nomination, according to a person familiar with their talks who spoke on the condition of anonymity because he was not authorized to discuss the endorsement process. Mr. Trump, who has taken particular pleasure in collecting the support of converted never-Trumpers, worked to clear the Wyoming field for Ms. Hageman, sending a fleet of aides to work for her and asking other candidates to drop out of the race after he made the endorsement.

The former president has for months focused on ousting Ms. Cheney. His aides and his son Donald Trump Jr. tried unsuccessfully in March to change Wyoming’s election law in a way that would have hurt the congresswoman’s re-election prospects.

Ms. Hageman, Mr. Trump said in his endorsement, “is all in for America First.”

It took years for Ms. Hageman to become an unabashed Trump supporter.

When she ran for Wyoming governor in 2018, Mr. Trump endorsed Foster Friess, a billionaire conservative donor who had backed Mr. Trump’s 2016 effort. Mr. Friess, who died in May, finished second to Mark Gordon, who was the state treasurer and is now Wyoming’s governor. Ms. Hageman placed third.

Ms. Hageman was known for her penchant to attack fellow Republican candidates in debates. She did not invoke Mr. Trump or his campaign themes in her television advertising.

“She was talking about state issues then, not anything federal,” said Diemer True, a former Wyoming State Senate president who also served as chairman of the Wyoming Republican Party.

In 2020, Ms. Hageman ran for office again, seeking one of Wyoming’s two posts as members of the Republican National Committee. This time, she aligned herself with Mr. Trump against Barbara Cubin, a former congresswoman backed by party moderates. Ms. Hageman prevailed at a virtual state party convention, 152 votes to 105.

Kitty Bennett contributed research.