Asked by a debate moderator on Tuesday why Gov. Tim Walz of Minnesota had said that he was in Hong Kong during the Tiananmen Square massacre in June of 1989, when he had in reality been in his home state of Nebraska, Mr. Walz said he was “a knucklehead at times.”

“All I said on this was, I got there that summer, and misspoke on this,” Mr. Walz added, when pressed to explain why he has maintained, for years, that he was in Hong Kong during the anti-government demonstration and entered China shortly afterward.

Mr. Walz tried to dismiss the misstatement as insignificant, saying he sometimes gets “caught up in the rhetoric.” He then pivoted to assert that his work as a teacher, congressman and governor was evidence that his community trusted his record despite his missteps.

Mr. Walz has long said that he was in Hong Kong on June 4, 1989, the day that Chinese soldiers killed hundreds of protesters in Tiananmen Square. He has said that he entered mainland China shortly after, even as others chose not to travel there, because he wanted to forge ahead with his yearlong teaching stint in the country — framing it as a courageous act.

“My thinking at the time was, what a golden opportunity to go tell, you know, how it was,” Mr. Walz told the podcast “Pod Save America” in February. “And I did have a lot of freedom to do that. Taught American history and could tell the story.”

But Mr. Walz was not in Hong Kong. He was in Nebraska until that August, when he left for China, according to news reports from the time. The timeline of his trip was first questioned by Minnesota Public Radio on Monday. His campaign did not provide an explanation.

Republicans have pounced on the news, pointing to it as another of a series of exaggerations and misstatements Mr. Walz has made, both large and small, that have surfaced since he was named Vice President Kamala Harris’s running mate.

Those include a comment he made in 2018 about “weapons of war that I carried in war” as a member of the National Guard, when he never served in combat. He has also implied that he and his wife used in vitro fertilization to start their family. In fact, the couple used a different treatment, intrauterine insemination.