UAW president Shawn Fain called on workers at Ford and General Motors plants in Chicago and Michigan to join the strike at noon on Friday.
“I'm calling on Ford's Chicago assembly plant to stand up and go on strike, and I'm calling on GM's Lansing Delta Township to stand up and go out on strike,” AFP reported Mr Fain as saying during a message live-streamed on YouTube.
“Our courageous members at these two plants are the next wave of reinforcements in our fight for record contracts.”
Workers began their strike last week demanding wage increases, shorter hours, better retirement benefits and other improvements.
More than 18,000 workers are already on strike at 41 plants across 21 states.
The strike has added to the stakes for the 2024 presidential election, with both President Joe Biden and former president Donald Trump wooing car workers.
Mr Biden joined Mr Fain and others on the picket line earlier this week, urging them to continue fighting for better pay.
“UAW, you saved the car industry back in 2008 and before, you made a lot of sacrifices, you gave up a lot when companies were in trouble,” he said.
The White House did not say if Mr Biden supports the 40 per cent wage increase that UAW is after.
Meanwhile, Mr Trump addressed workers at a non-union car parts store in Clint Township. Mr Fain did not join him.
Mr Biden narrowly beat Mr Trump – the 2024 Republican front-runner – in the state in 2020.
The UAW, which endorsed Mr Biden in the previous election cycle, has not endorsed any 2024 candidate yet.