With some people hoisting signs reading "Country over Party", Harris told the crowd on Thursday that "people of every party must stand together" to reject Trump, citing his refusal to accept the results of the 2020 election and his failure to quell the insurrection of January 6, 2021.
It was an improbable moment, a Democratic nominee giving a nod to a rival party member and to the origins of the opposing party in the closing weeks of a presidential campaign, and it demonstrated how much Harris is attempting to win over moderate and crossover Republican voters.
"He refused to accept the will of the people and to accept the results of an election that was free and fair," Harris said of Trump.
"The president of the United States must not look at our country through the narrow lens of ideology or party partisanship or self-interest," she added. "Our nation is not some spoil to be won. The United States of America is the greatest idea humanity ever devised."
Democrat candidate Kamala Harris is attempting to win over moderate and crossover Republican voters. (AP PHOTO)
Cheney is one of Trump's most ardent antagonists. She is the daughter of former Republican Vice President Dick Cheney and was the top Republican lawmaker on the House committee that investigated the January 6, 2021, insurrection, earning Trump's disdain and effectively exiling herself from her own party.
"Violence does not and must never determine who rules us. Voters do," Cheney told the crowd as she recounted Trump refusing to act as he watched the violent attack on television. Someone in the crowd yelled "coward!" Others booed.
Adding to the surreal nature of the event, the crowd cheered references to Dick Cheney and to another Republican former vice president: Mike Pence, who refused to bow to pressure from Trump and attempt to stop the certification in Congress of Biden's 2020 victory.
In an interview Thursday night with Fox News Channel, Trump said of Harris and Cheney: "I think they hurt each other. I think they're so bad, both of them."
Cheney lost her Wyoming seat to a Trump-endorsed candidate two years ago and endorsed Harris, the Democratic nominee, in September. The two women appeared together in Ripon, home to a white schoolhouse where a series of meetings held in 1854 to oppose slavery's expansion led to the start of the Republican Party.
"I know that she loves our country, and I know she will be a president for all Americans," Cheney said of Harris. Noting that she herself remains conservative, Cheney said she was "honoured to join her in this urgent cause".
Harris is on a two-day Wisconsin and Michigan swing, while Trump was in Michigan on Thursday as both candidates grapple for wins in the "blue wall" battleground states, which also include Pennsylvania.
Donald Trump spoke at a rally in Michigan as both sides chase votes in so-called "blue wall" states. (AP PHOTO)
While Cheney and Harris spoke, the former president took his social media site to say Democrats and prosecutors have lied about the "huge crowd of Patriots gathered in Washington, D.C. on January 6th".
That was a far cry from President Joe Biden's reaction. Arriving back at the White House after touring damage from Hurricane Helene in Georgia and Florida, Biden said of Cheney: "She made one of the most consequential speeches I've ever heard. She has character."
"I know her dad," Biden added. "We argue like hell, but I always admired his courage and honesty. What she did not took only political courage, but physical courage."
Harris' visit to Wisconsin came a day after a federal judge unsealed a 165-page court filing outlining prosecutors' case against Trump for his attempt to overturn his 2020 election defeat. Trump has pleaded not guilty to charges of conspiracy and obstruction.
