Liz Cheney delivers defiant speech on House floor ahead of vote to oust her from G.O.P. leadership.

Representative Liz Cheney of Wyoming delivered a defiant last stand hours before facing a vote to purge her from House Republican leadership for her outspoken repudiation of former President Donald J. Trump’s election lies, declaring on Tuesday on the House floor that she would not sit back quietly as her party undermined democracy.

Ms. Cheney, who is facing a vote Wednesday morning that is almost certain to succeed in ousting her from House Republicans’ No. 3 post, declared on Tuesday night that the nation was facing a “never seen before” threat in a former president who provoked the Jan. 6 attack and who “has resumed his aggressive effort to convince Americans that the election was stolen from him.”

“Remaining silent and ignoring the lie emboldens the liar,” Ms. Cheney said. “I will not participate in that. I will not sit back and watch in silence, while others lead our party down a path that abandons the rule of law and joins the former president’s crusade to undermine our democracy.”

The Wyoming Republican’s remarkable broadside illustrated how she has unrepentantly received the effort to dethrone her, and offered herself as a cautionary tale in what she is portraying as a battle for the soul of the G.O.P. She has cast her almost certain expulsion from the leadership ranks as a “turning point” for her party and told allies that the leadership post is simply not worth having if it requires her to lie.

In her speech Tuesday night, Ms. Cheney, a former State Department official, invoked the parallels between what unfolded at the Capitol on Jan. 6 and her work in authoritarian countries to explain why she was so determined to publicly condemn the attempted insurrection.

“Those who refuse to accept the rulings of our courts are at war with the Constitution,” Ms. Cheney said. “Our duty is clear. Every one of us who has sworn the oath must act to prevent the unraveling of our democracy. This is not about policy. This is not about partisanship. This is about our duty as Americans.”