Barely 24 hours before the contract deadline, the United Auto Workers leader said Wednesday that his members were prepared for a strike against the three Detroit automakers — first at a limited number of factories, with the walkout expanding if talks remain bogged down.

The U.A.W. president, Shawn Fain, also ruled out any extension of the existing four-year contracts with General Motors, Ford Motor and Stellantis after they expire on Thursday night. “September 14 is a deadline, not a reference point,” he declared in an address to union members on Facebook Live.

He said the initial strike locations would be “limited and targeted,” and would be communicated to members on Thursday night ahead of a Friday walkout.

Striking at even a handful of plants would disrupt the automakers’ production while ensuring that a large portion of the 150,000 U.A.W. members at the three companies continued to work and receive paychecks.

The union plans to pay striking workers $500 per week and cover the cost of their health insurance premiums. The union has a strike fund of $825 million, which would cover payments to workers in a full strike against all three companies for about three months.

In its initial proposal to the companies, the union demanded a 40 percent increase in wages over four years, on the premise that pay packages of the companies’ chief executives have on average risen that much over the last four years. The union has also sought regular cost-of-living adjustments that would nudge wages higher in response to inflation.

The union is also seeking pensions for all workers, improved retiree benefits, shorter work hours and an end to a tiered wage system that starts new hires at about half the top U.A.W. wage of $32 an hour.

The companies — each negotiating separately with the union — have made counterproposals for wage increases of roughly half what the union is asking, according to Mr. Fain, and have essentially rejected all of the other demands.

The companies did not immediately respond to requests for comment on Mr. Fain’s announcement and his characterization of the negotiations.

Erik Gordon, a business professor at the University of Michigan who follows the auto industry, said a strike was very likely. “I think they can reach an agreement on wages,” he said, “but these other issues are complicated and can’t be resolved in the last 36 hours by splitting the difference.”

The prospect of a large-scale strike comes as the automakers are reaping near-record profits but also contending with the transition to electric vehicles. G.M., Ford and Stellantis — the parent of Chrysler — are investing tens of billions of dollars to develop new technologies and electric models, build new battery plants, and retool older factories.

This is a developing story. Check back for updates.

Kurtis Lee contributed reporting.