Dick Cheney has branded Donald Trump the greatest “threat to our republic”, in a new campaign ad for his daughter, Liz Cheney, who is running for re-election in Wyoming.

“In our nation’s 236-year history, there has never been an individual who is a greater threat to our republic than Donald Trump,” said Cheney, who served as vice-president for two terms under George W Bush.

Cheney said: “He tried to steal the last election using lies and violence to keep himself in power after the voters had rejected him.

“He is a coward. A real man wouldn’t lie to his supporters. He lost his election, and he lost big. I know it, he knows it, and deep down I think most Republicans know it.”

Cheney went on to speak about how proud he was of his daughter “for standing up to the truth, doing what’s right, honoring her oath to the constitution when so many in our party are too scared to do so”.

The one-minute ad featured the elder Cheney’s sharpest public attacks against Trump to date. Best known as the most powerful vice-president in American history, and a major figure in leading the US to war in Iraq, he has taken to defending his daughter in her fight against Trump.

“There’s nothing more important she will ever do than lead the effort to make sure Donald Trump is never near the Oval Office. And she will succeed,” he said in the ad.

The younger Cheney has been widely praised from liberals as vice-chairwoman of the House select committee investigating the January 6 attack. Cheney has been one of Trump’s most pointed critics, accusing him of violating the constitution for his role in the insurrection.

In return, she has been largely ostracized from her party. Cheney faces an uphill re-election battle against the Trump-backed candidate Harriet Hageman, who maintains that the 2020 election was stolen.

“Liz Cheney has long forgotten she works for Wyoming (or perhaps she never knew), not the Radical Democrats,” Hageman tweeted on Thursday. “Wyoming deserves a Congresswoman who will represent us AND our conservative values. It’s time to retire elitist Liz Cheney.”

Though Cheney has at least a million dollars more in donations to her campaign against Hageman, she was 22 points behind Hageman in a July poll conducted by the Casper Star-Tribune.

In an interview with CNN on Thursday, Cheney said she does not expect to lose on 16 August.

“I really believe that the people of Wyoming fundamentally understand how important fidelity to the constitution is – understand how important it is that we fight for those fundamental principles on which everything else is based,” she said.