President Joe Biden, who suspended his campaign last month to endorse Harris, said in a statement that the Democratic nominee would be “the strongest defender of our individual freedoms and democracy.”
Pennsylvania Governor Josh Shapiro, who is considered a possible running mate for Ms. Harris, also attended the rally.
He has faced sharp criticism from the left for his support of Israel and for his handling of university protests sparked by the war in Gaza.
Some of Trump’s advisers expressed relief that Mrs. Harris did not select Shapiro, citing concerns that he could be a key candidate in a key state like Pennsylvania.
Before becoming governor, Mr. Waltz represented a Republican-leaning district in the U.S. Congress for 12 years.
He won the seat in 2006, the only Democrat to hold it in this mostly rural district in the past three decades.
Mr. Waltz is a native Nebraska citizen, the son of a school administrator and a stay-at-home mother.
He grew up farming and hunting, enlisted at age 17, and served in the Army National Guard for 24 years.
Young Mr. Waltz also taught secondary school students. He first taught in China for a year. He said he has been to China about 30 times. He speaks a little Mandarin.
His wife, Gwen Whipple, was a teaching colleague and brought him to her native Minnesota, where he taught social studies and geography and coached football.
