Multiple cars travelling in the motorcade carrying Minnesota Governor Tim Walz, the Democratic candidate for vice-president, crashed in Milwaukee.
Mr Walz was not hurt.
The crash happened around 13:00 local time (18:00 GMT) on Monday. The cars involved were at the rear of the motorcade and were carrying members of the press who travel with Mr Walz.
Reporters said they were “violently thrown forward” after being hit from behind, sending their vehicle into the car in front of them. At least one person was injured.
It is unclear what caused the crash, which occurred on Interstate 794.
Vice-President Kamala Harris, his running mate, phoned Mr Walz to check that he was OK, a White House official told CBS, the BBC’s US partner. Mr Walz said he also received a call from President Joe Biden.
One person appeared to have broken an arm and was being treated by medics, according to a pool reporter travelling with Mr Walz.
The affected vehicles pulled over to the side of the road as Mr Walz continued ahead to a scheduled appearance.
Mr Walz addressed the incident in his remarks at a Milwaukee Labor Day event shortly after the crash.
“Some of my staff and members of the press that were travelling up with us were involved in a traffic accident on the way here today. We’ve spoken with the staff,” he told the crowd. “I’m relieved to say that with a few minor injuries, everybody’s going to be okay.”
Mr Walz thanked the Secret Service and local first responders “for their quick reaction to help”.
Republican vice-presidential nominee JD Vance also responded to the reports, writing on X, formerly known as Twitter, “Hoping everyone’s OK.”
It’s not the first time the campaign has faced a motorcade-related crash.
Last week, a Georgia policeman crashed his motorcycle while travelling with the campaign convoy.
The officer, Savannah Police Corporal David Bates, was the only person involved in the crash and sustained “serious injuries”.
NPR reported that motorcade vehicles are often driven by campaign staff who may or may not have professional driving experience.
