It was an exercise in unsubtle and unlikely campaign optics: a Democratic vice president who is running for the presidency. A Republican former congresswoman who is the daughter of a staunchly conservative vice president. A small city known as the birthplace of the Republican Party in the middle of a battleground state.

On Thursday, Vice President Kamala Harris and former Representative Liz Cheney of Wyoming, the most prominent Republican to endorse her campaign, traveled to Ripon in central Wisconsin where meetings in 1854 helped form the Republican Party. Just a mile away from a one-room schoolhouse where those gatherings were held, the pair tore into former President Donald J. Trump for his role in igniting a riot at the Capitol, and they warned of the threat he poses to democracy should he return to power.

Ms. Cheney said that, in November, putting patriotism ahead of partisanship should not merely be an aspiration — “it is our duty.”

Her remarks, delivered with an air of somber restraint, were as much a public indictment of Mr. Trump as they were an endorsement of Ms. Harris. Calling his candidacy “a threat unlike any we have faced before,” she called on conservatives to join her in an “urgent cause” to elect Ms. Harris and to reject what she called the former president’s “depraved cruelty.”

“I know that she will be a president who will defend the rule of law,” Ms. Cheney said of Ms. Harris, “and I know that she will be a president who can inspire all of our children and, if I might say so, especially our little girls.”

The joint appearance was one of the starkest examples to date of how Ms. Harris has endeavored to pitch herself as a unifying president who values pragmatism over partisanship. Her overarching goal is to win over moderate and independent voters who will be crucial to delivering her a decisive victory.

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