In one of her first campaign appearances since former President Donald J. Trump was convicted of falsifying business records, Vice President Kamala Harris is expected to sharply criticize him on Saturday as a “cheater” who believes himself above the law and to argue that he should be disqualified for the presidency.

Ms. Harris, who is set to headline a state Democratic Party dinner in Detroit, plans to rebuff Mr. Trump’s false claims that his trial, like the 2020 presidential election, was “rigged” and to defend the judicial process behind his felony conviction, according to excerpts from her prepared remarks obtained by The New York Times.

In the excerpts, Ms. Harris, the former top prosecutor of California, accuses Mr. Trump of attacking “the foundations of our justice system.” She says that the former president was convicted by a jury of 12 Americans who were selected in part by his defense team, and that his lawyers had a chance to present their side of the evidence.

“You know why he complains? Because the reality is cheaters don’t like getting caught,” the excerpts state.

The event will take place in a battleground with heightened stakes. President Biden won Michigan’s primary in February, 81 percent to 13 percent, prevailing over a movement that urged Democrats to vote “uncommitted” on the ballot in protest of his support for Israel. But more than 100,000 voters took that stance against him, among them progressives, young people and many in the state’s large and politically active Arab American community. Mr. Biden’s campaign has also been seeking to shore up its support among Black voters in cities like Detroit.

Mr. Trump won Michigan by nearly 11,000 votes in 2016, and lost it to Mr. Biden by more than 150,000 votes in his 2020 re-election bid. Mr. Trump focused on the voting in Michigan in his efforts to subvert the 2020 election.