The United Auto Workers said on Tuesday that the union would expand its strike against three U.S. automakers on Friday if it was unable to make substantial progress in contract talks with them.

Nearly 13,000 U.A.W. members walked off the assembly lines at three plants last Friday, one each at the three companies — General Motors, Ford Motor and Stellantis, the parent of Chrysler. The union has demanded a 40 percent wage increase over four years, better benefits and other changes. The automakers, which are based in or have a big presence in Michigan, have offered raises of about half as much.

In a video posted on Facebook on Tuesday, the union’s new president, Shawn Fain, said workers could walk out of more plants at the end of this week.

“If we don’t see serious progress to noon Friday, Sept. 22, more locals will be called on to stand up and go on strike,” he said. “We’re going to keep hitting the companies where we need to.”

Separately on Tuesday, Mr. Fain responded to criticism by former President Donald J. Trump, who is expected to visit the Detroit area next week.

“Every fiber of our union is being poured into fighting the billionaire class and an economy that enriched people like Donald Trump at the expense of workers,” Mr. Fain said. “We can’t keep electing billionaires and millionaires that don’t have any understanding of what it is like to live paycheck to paycheck and struggle to get by and expecting them to solve the problems of the working class.”

In an interview on NBC’s “Meet the Press” last weekend, Mr. Trump said Mr. Fain and the union were “failing” workers in the shift to electric vehicles that has been championed by President Biden.

“The autoworkers are being sold down the river by their leadership,” he said, adding: “All of these cars are going to be made in China. The electric cars, automatically, are going to be made in China.”

Mr. Biden has expressed support for the striking workers, although the U.A.W. has not endorsed his re-election thus far. The union has long backed Democratic presidential candidates, but some of its members supported Mr. Trump in the last two elections.

The union and the companies, which are engaged in three separate negotiations, remain far apart. The companies have offered raises of about 20 percent, but Mr. Fain has said that doesn’t go far enough to make up for the impact of inflation and concessions the union made over the last 15 years.

The union also wants pensions to cover more workers, company-paid health care for retirees, shorter working hours and measures that make it harder for the companies to close plants in the United States. The automakers have rejected most of those other demands.

In statements and interviews, auto executives have said meeting all of the union’s demands would put them at a severe competitive disadvantage to nonunion plants operated by Tesla and foreign automakers such as Toyota and Volkswagen. G.M., Ford and Stellantis already have higher labor costs than most nonunion car companies.

The three automakers have said they cannot afford substantial raises and new benefits because they are investing tens of billions of dollars to develop electric vehicles and build battery plants.