Representative Jim Jordan, an Ohio Republican, said that Mr. Walz had “let rioters and looters burn a police station to the ground.” Representative Andy Biggs of Arizona said Mr. Walz had “let his state burn, and Kamala raised money to bail these criminals out of jail.”

“This is the America that the radical left wants,” Mr. Biggs said in a social media post.

Mr. Walz was known as a moderate during his years in the House, but Republicans were quick to seize on the liberal laws he has passed with the help of a friendly Democratic-controlled Minnesota Legislature.

Among Senate Republicans, Mike Lee of Utah said the Harris-Walz represented “the most radical ticket in American history,” while Marco Rubio of Florida said the choice of Mr. Walz showed that “the lunatic left has completed its takeover” of the Democratic Party.

“To characterize him as left is so unreal,” Ms. Pelosi, a California Democrat, said on MSNBC.

The super PAC supporting Mr. Trump, MAGA, Inc., attacked Mr. Walz for his executive order protecting access to transition care in his state.

The issue has been a lightning rod for both parties. When Mr. Walz signed the order, he singled out a ban on transition care for minors in South Dakota, which had been approved by Gov. Kristi Noem, at one point a potential running mate for Mr. Trump.

Senator JD Vance of Ohio, whom Mr. Trump ultimately chose as his vice-presidential candidate, has proposed a similar federal ban in Washington.

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