Liz Cheney embraces her role in the Jan. 6 inquiry in a closing campaign ad.

Representative Liz Cheney of Wyoming is highlighting her role as the top Republican on the Jan. 6 committee in a closing ad for her all but doomed re-election campaign, as polls show her badly trailing her Trump-backed opponent, Harriet Hageman, just five days before the primary.

But the nearly two-and-a-half-minute ad released online Thursday appeared aimed as much at a national audience as at the Republican primary voters in Wyoming who will decide the fate of Ms. Cheney, the state’s lone member of the House.

“The lie that the 2020 presidential election was stolen is insidious,” Ms. Cheney said as the ad opens. “It preys on those who love their country. It is a door Donald Trump opened to manipulate Americans to abandon their principles, to sacrifice their freedom, to justify violence, to ignore the rulings of our courts and the rule of law.”

Ms. Cheney, who has been vilified by former President Donald J. Trump and many of his supporters, defended the work of the special House committee that is investigating the 2021 attack on the Capitol and efforts by Mr. Trump to overturn the 2020 election results.

Ms. Cheney, the vice chairwoman of the Jan. 6 committee, has acknowledged her political peril. A poll released on Thursday by the University of Wyoming’s Wyoming Survey and Analysis Center showed Ms. Cheney trailing Ms. Hageman by nearly 30 points.

She is the last of the 10 House Republicans who voted for Mr. Trump’s impeachment to stand before voters in a primary this year. Three have lost: Representatives Jaime Herrera Beutler of Washington, Tom Rice of South Carolina and Peter Meijer of Michigan. Two others survived their primaries, and four declined to seek another term.

Titled “The Great Task,” the ad is being promoted on social media, but is not appearing on television, according to Jeremy Adler, a campaign spokesman for Ms. Cheney.

In the ad, Ms. Cheney described Mr. Trump’s false claims of election fraud as his legacy and said that the nation has an obligation to hold those responsible for fomenting violence.

“History has shown us over and over again how these types of poisonous lies destroy free nations,” Ms. Cheney said of those insisting that Mr. Trump won the election. “No one who understands our nation’s laws, no one with an honest, honorable, genuine commitment to our Constitution would say that. It is a cancer that threatens our great republic.”

Ms. Cheney did not mention Ms. Hageman by name in her ad, but drew a comparison between her opponents in Wyoming and election-denying candidates across the nation. Last week, Ms. Hageman repeated Mr. Trump’s false claim that the election was rigged.

Tim Murtaugh, an adviser for Ms. Hageman’s campaign, accused Ms. Cheney of abandoning Wyoming. “This video is basically an audition tape for CNN or MSNBC,” he said.

Ms. Cheney’s renunciation of Mr. Trump — and her vote to impeach him last year — have already come at a political price. The Wyoming Republican Party censured her in February 2021, a month after Ms. Cheney’s impeachment vote. House Republicans later ousted Ms. Cheney as the party’s No. 3 leader in the chamber, replacing her with Representative Elise Stefanik, a Trump loyalist from New York.

As the ad closed, Ms. Cheney said that she would always seek to preserve peaceful transitions of power, “not violent confrontations, intimidation, and thuggery,” and added, “where we are led by people who love this country more than themselves.”