WASHINGTON — The last time Representative Liz Cheney of Wyoming was staring down an attempt to purge her from House Republican leadership, as punishment for having voted to impeach former President Donald J. Trump, she whipped the vote herself, privately calling almost every member of her party to lock up support until she was confident the challenge would fail.

But three months later, as she prepares to face a fresh vote on Wednesday to force her out of the No. 3 leadership post — this one orchestrated by party leaders who have grown exasperated with her outspoken repudiation of Mr. Trump’s election lies — Ms. Cheney has no such illusions of hanging on.

On the precipice of what looks to be an almost certain ouster, rather than barreling through her Rolodex of lawmakers or rallying her extensive network of backers off Capitol Hill to try to cling to her job, Ms. Cheney has defiantly embraced her downfall, offering herself as a cautionary tale in what she is portraying as a battle for the soul of the G.O.P.

She has spent the hours leading up to the vote continuing to blister Mr. Trump and his falsehoods about the 2020 election, framing her expulsion from the leadership ranks as a “turning point” for her party and telling allies that the leadership post is simply not worth having if it requires her to lie.

“I think Liz understands it’s not worth selling your soul for No. 3 in the minority,” said Barbara Comstock, a former Republican congresswoman from Virginia and a friend of Ms. Cheney’s. “She’s just not going to do that.”

Her defiant exit illustrates Ms. Cheney’s determination to continue her blunt condemnation of Mr. Trump and her party’s role in spreading the false election claims that inspired the Jan. 6 attack on the Capitol. It has also highlighted how Republican leaders, even in their eagerness to rebuild their party after the riot and Mr. Trump’s stormy departure from the White House, have tethered themselves to those lies as a matter of survival.

As a replacement for the Wyoming Republican, leaders have united behind Representative Elise Stefanik of New York, a onetime moderate whose fealty to Mr. Trump and backing for his false narrative of a stolen election have earned her broad support from the party’s rank-and-file that Ms. Cheney, as a lifelong conservative, no longer commands. It represents a remarkable arc for the daughter of a conservative dynasty who was once spoken of as a future speaker and now stands on the cusp of being relegated to the political wilderness.

Ms. Cheney’s allies say she views the ouster as part of an existential battle and intends to keep up her criticism even from exile in the rank and file. The short-term political consequences will almost surely work against her, both in Washington and at home.

Ms. Cheney is under attack in Wyoming, where conservatives who have sought to exploit her antagonism of Mr. Trump are hoping to unseat her in a primary. And any presidential aspirations she may have nursed appear certain to be delayed — if not dashed — given her decision to take on a figure whom the Republican base reveres.

The drama that unfolded before the vote on Tuesday underscored the iron grip Mr. Trump still has on the party, as hard-right Republicans began to publicly argue that Ms. Stefanik was not sufficiently conservative or supportive enough of the former president to lead the conference.

In a memo circulated by Representative Chip Roy of Texas that was reported by Politico, he tore into Ms. Stefanik and accused the Republican leaders who have championed her of “rushing to coronate a spokesperson whose voting record embodies much of what led” to Republicans’ drubbing in the 2018 midterm elections.

He also denounced Ms. Cheney for “unhelpfully engaging in personal attacks and finger-wagging towards President Trump rather than leading the conference forward.” Mr. Roy’s letter reflected a determination among conservatives, who led the first, unsuccessful effort to oust Ms. Cheney in February, to exert their will on the party’s message.

Even outspoken allies of Ms. Cheney earlier this year appeared ready to abandon her. Representative Mike Gallagher of Wisconsin, a fellow defense hawk, told The Milwaukee Journal Sentinel that he planned to vote to oust Ms. Cheney from leadership. In backing Ms. Cheney in February, Mr. Gallagher had warned that “we must be a big tent party or else condemn ourselves to irrelevance.”

But on Tuesday, Mr. Gallagher said in a statement: “House Democrats under Speaker Pelosi have been ruthless in advancing their radical progressive agenda, and Representative Cheney can no longer unify the House Republican conference in opposition to that agenda.”

Instead, Ms. Cheney found a set of unlikely allies rallying to her side: Democratic leaders.

“Liz Cheney spoke truth to power, and for that she is being fired,” said Senator Chuck Schumer of New York, the majority leader.

Representative Steny H. Hoyer of Maryland, the No. 2 House Democrat, chimed in to say he found it “sad” to watch Republican leaders pledge fealty to “such a dysfunctional leader as Donald Trump.”

Similar praise from Democrats had infuriated Republicans in January, after Ms. Cheney released a lengthy, unsparing statement announcing that she would vote to impeach Mr. Trump. Democrats quoted liberally from it on the House floor, to the dismay and embarrassment of Republicans, some of whom felt that the Wyoming Republican was grandstanding to further her political ambitions.

Ms. Cheney’s detractors have accused her of continuing to do so.

“I think she made a calculated decision that she would rather be a martyr than try to accommodate her own conference,” Newt Gingrich, a former Republican speaker, told Fox News’s Maria Bartiromo this week. “She has every right as an individual member to say and do what she wants to. But she has no right as the chair of the conference to take the power and the prestige that the conference has given her and use it to undermine the conference.”

Ms. Cheney, however, has repeatedly warned that trying to avoid talking about the riot and Mr. Trump’s false election claims will not only further alienate would-be Republican voters, but also cause “profound long-term damage” to the country. A former State Department official, she has invoked the parallels between what unfolded on Jan. 6 and her work in authoritarian countries to explain why she was so determined to publicly condemn the attempted insurrection.

“History is watching. Our children are watching,” Ms. Cheney wrote in a scathing Washington Post op-ed last week. “We must be brave enough to defend the basic principles that underpin and protect our freedom and our democratic process. I am committed to doing that, no matter what the short-term political consequences might be.”