There was a time in 2016 when Elise Stefanik, now the third-ranking Republican in the House, was so disgusted by Donald Trump, she would barely mention his name. Today he proudly refers to her as “one of my killers.”She proved that again last month. In an effort to undermine confidence in the select committee investigating the violent assault on the Capitol, Ms. Stefanik said, “This is not a serious investigation. This is a partisan political witch hunt.” The committee, she said, is “illegitimate.” The hearings did not change her mind. In mid-July, before the final session planned for the summer, she referred to the committee as a “sham” and declared that “it is way worse than the impeachment witch hunt parts one and two.”Maybe Ms. Stefanik was continuing to discredit the House committee because the evidence it has produced from Trump insiders — and the compelling way the evidence has been presented — has inflicted staggering damage on Mr. Trump, even though it might not prevent him from winning the Republican presidential nomination for a third straight time. Ms. Stefanik has failed in her efforts to sabotage the committee, but it’s not for lack of trying.Ms. Stefanik’s fealty to Mr. Trump is so great that some of his advisers are mentioning her as a potential vice-presidential candidate if he runs in 2024, which he and his advisers are strongly hinting he will do.The transformation of Ms. Stefanik, who is 38, is among the most dramatic and significant in American politics. Her political conversion is a source of sadness and anger for several people I spoke to who were colleagues of hers — as I was in the White House of George W. Bush although I did not work with her directly — and who were, unlike me, once close to her. To them, Ms. Stefanik’s story is of a person who betrayed her principles and her country in a manic quest for power.Born in upstate New York, Ms. Stefanik graduated from Albany Academy for Girls and Harvard, after which she joined the Bush administration as a staff member for the Domestic Policy Council and later in the office of the White House chief of staff. She worked for the 2012 presidential campaign of Tim Pawlenty before overseeing debate preparation for the Republican vice-presidential nominee, Paul Ryan.Ms. Stefanik was elected to the House of Representatives in 2014, becoming at the time the youngest woman elected to Congress. In January 2017, Ms. Stefanik became a co-chair of the Tuesday Group, made up of moderate Republicans who served as a counterbalance to the right-wing Freedom Caucus, which was co-founded two years earlier by Mark Meadows, who later became chief of staff to Mr. Trump when he was president. At the time, Ms. Stefanik was viewed as pragmatic and highly competent, a member of the establishment wing of the Republican Party and a person Democrats felt they could do business with.But within a matter of a few years, all that changed, with Ms. Stefanik referring to herself as “ultra-MAGA and proud of it.” Because of her previous beliefs, she had to reassure Trump supporters. So last year she appeared on the podcast of the right-wing provocateur Steve Bannon, a popular figure with the Republican base who served as Mr. Trump’s chief strategist, to make her case to replace Liz Cheney as chair of the House Republican Conference. Ms. Stefanik was supported in her effort to supplant Ms. Cheney by Mr. Trump, who issued a statement giving his “complete and total endorsement” to Ms. Stefanik. And understandably so. She voted to invalidate the 2020 election and has repeated his false claims about election fraud.I spoke to several people who have worked with Ms. Stefanik over the years — some who were willing to speak on the record, others who insisted on anonymity so that they could speak candidly to help me understand what had happened to her and what her rise to power and celebrity status says about the modern-day Republican Party.Those who know Ms. Stefanik well consider her smart and talented but hollow at the core. She is an individual of “ambition unmoored to principle,” as Barbara Comstock, a Virginia Republican who served in Congress with Ms. Stefanik, told me.Margaret Hoover, the host of “Firing Line,” who worked in the Bush administration with Ms. Stefanik and was consulted by her before her run for Congress in 2014, described her as a person of “sheer ambition and not principled at all.” Another Republican — a former member of Congress who served with Ms. Stefanik and worked closely with her — also spoke to me of her towering ambition, invoking Icarus, the Greek mythological character. “She’s flying too close to the sun,” said this person, who requested anonymity in order to speak openly about her transformation.People who worked with Ms. Stefanik say that during the 2016 campaign, her reaction to Mr. Trump was quite negative and that she was particularly disgusted by his attitude toward women. She considered Mr. Trump’s comments on the “Access Hollywood” tape sickening — and like many others, she assumed he would lose the election. He didn’t, of course, and Ms. Stefanik, like so many other Republicans, began the process of accommodation. Soon hers would be complete.When the Republican Party was a George W. Bush party, she was a Bush Republican. When the Republican Party became a Trump party, she was a Trump Republican. Former colleagues of hers will tell you she meant it then and she means it now. She’s a person who takes her views from the place she finds herself — and the place she finds herself today is in a pro-Trump district and in a thoroughly Trumpified party.Several people I spoke to about Ms. Stefanik mentioned a couple of key moments in her journey to MAGA world. The first was an August 2018 visit by Mr. Trump to Fort Drum, an Army base that has a substantial economic and political impact in New York’s 21st Congressional District, which she represents. The large crowd the president drew and the enthusiasm he generated registered with Ms. Stefanik, who welcomed him. “Elise stood in front of the MAGA Trump crowd and decided to shed her old self and follow instead of lead,” Ms. Comstock told me. “It was the beginning of the end.”But the most important inflection point was the first impeachment trial of Mr. Trump, in 2019. Ms. Stefanik accused Adam Schiff, the Democratic chairman of the committee overseeing the impeachment trial, of trying to silence Republicans and clashed with the former U.S. ambassador to Ukraine Marie L. Yovanovitch, whose testimony about Mr. Trump was damning.Mr. Trump was thrilled by Ms. Stefanik’s performance; he called her a “new Republican star” on Twitter. Her life changed after that. She became much better known and was able to raise a lot of money from her new position and with her new posture.“She became a celebrity,” I was told by a Bush administration colleague of Ms. Stefanik’s who requested anonymity so this individual could speak freely about what is still a very sensitive subject. Until that point, this person said, “she hadn’t gotten fully on board the Trump train. Then she was put into first class, and she couldn’t get off. And first class is pretty plush.”I reached out twice to Ms. Stefanik’s communications director, seeking comment from the congresswoman, but received no response. Ms. Stefanik, in defending herself, has argued that she’s reflecting the views of a majority of the people in her district, and she is. Mr. Trump carried her district by 14 percentage points in 2016. “I represent farmers, manufacturers and hard-working families who want someone who stands up for them, and President Trump spoke to those people,” she told Mr. Bannon on his show.But even if you believe that the job of an elected representative is to vote according to the will of the voters rather than to owe voters one’s “judgment and conscience,” as the British parliamentarian and conservative political theorist Edmund Burke famously put it, at some point carrying out the will of the voters can become indefensible. That is certainly true if it requires a member of Congress to support a seditious president.Looking at what happened with Ms. Stefanik is sad and disturbing because people who know her say she knows better. She was willing to be shaped by circumstances, even when circumstances drove her to ugly places and to embrace conspiracy theories. Contrast this with Ms. Cheney, who was stripped of her position in the Republican leadership and replaced by Ms. Stefanik. Ms. Cheney represents the people of Wyoming on many issues that are important to them, but she drew a line when it came to a fundamental attack on our democracy. She wouldn’t cross that line. Ms. Stefanik did.Ms. Stefanik’s story is important in part because it mirrors that of so many other Republicans. They, like Ms. Stefanik, are opportunists, living completely in the moment, shifting their personas to advance their immediate political self-interests. A commitment to ethical conduct, a devotion to the common good and fidelity to truth appear to have no intrinsic worth to them. These qualities are mere instrumentalities, used when helpful but discarded when inconvenient.The politicians and former Bush administration officials I spoke to were worried that Republicans in Congress will conclude that Ms. Stefanik’s path to power is the one to emulate. The fast track to leadership is to enlist figures like Mr. Trump, Mr. Bannon and what one of my interlocutors called “the army of the base,” made up as it is of QAnon followers, Christian nationalists, right-wing talk radio aficionados and those who are determined to overturn elections.The Bush administration figure who worked with Ms. Stefanik told me that her move into MAGA-dom was illustrative because it was representative of a larger problem. “In isolation, Elise is not a particular malefactor. She’s more a symptom than a disease.” But, this individual said, she and other Republicans “could have made a difference if they had had collective courage.”They could have made the case against Mr. Trump’s malicious and unconstitutional conduct. They could have attempted to mold the sentiments of the Republican base in a healthy direction. But they refused.Never mind Ms. Stefanik. “I affix a lot of the blame on the dozens and dozens of Republican leaders who acquiesced in what they knew to be wrong,” this person said.During the Trump era, we saw a profound failure of leadership among Republican lawmakers when it came to calming down inflamed populist passions.Wise observers of politics have told me that what leadership does in a populist moment like ours is to distinguish between legitimate and illegitimate grievances. Leaders speak to legitimate grievances and channel them in constructive ways through policies. Demagogues elevate illegitimate grievances and speak to them in reckless ways. In populist times, good leaders tamp down on the bad and elevate the good. Ms. Stefanik and many, many others chose to elevate the worst.This has inflicted a grave cost on the political profession, making Americans even more cynical about the whole political enterprise. I hate to think about the message it sends younger people who are thinking about running for office.Someone who takes the route to power Ms. Stefanik has chosen “degrades and demeans public service,” Ms. Hoover told me. “Anyone who cares about our political system should find what she’s done so deeply offensive. We deserve better. Our country deserves better, and those who came before us deserve better.”At the end of my conversation with Ms. Comstock, I asked for her assessment of the game Ms. Stefanik is playing.“I do believe this catches up with people,” she said. “There might be what appears as a short-term benefit, but situations like this often spectacularly fail.”“I don’t view Elise’s story as a success story,” she added. “It won’t end well. Stories like this never do.”Peter Wehner (@Peter_Wehner) — a senior fellow at the Trinity Forum who served in the administrations of Presidents Ronald Reagan, George H.W. Bush and George W. Bush — is a contributing Opinion writer and the author of “The Death of Politics: How to Heal Our Frayed Republic After Trump.”The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. And here’s our email: letters@nytimes.com.Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram. More