Another 7,000 members of the biggest US automobile workers' union on Friday joined its already major strike against the country's "Big Three" Detroit car manufacturers.

The latest walkouts leave a total of some 25,000 workers on strike in 21 states — or around 17 percent of the United Auto Workers' membership.

UAW president Shawn Fain said the latest members to join the strike were at two plants in Chicago and Michigan, who downed tools at 1600 GMT.

"Our courageous members at these two plants are the next wave of reinforcements in our fight for record contracts," Fain said.

The industrial action is the first-ever joint strike at the major automakers — Ford, GM, and Chrysler producer Stellantis — in a push for higher salaries and other improvements.

Fain noted however that he would not call on additional members to suspend their activity at Stellantis due to "significant progress" on several points currently under discussion.

Fain mentioned a mechanism for adjusting salaries to the cost of living as well as the right to strike in the event of a site closure or relocation announcement by the manufacturer.

The union has asked for a 40 percent pay raise over the next four-year contract, while the three companies have been offering raises of around 20 percent.

It's also been negotiating to represent workers at battery plants planned by Ford, which the auto giant has so far resisted.

UAW has used the targeted strike, which began on September 15, as a bargaining chip, ramping up the pressure on the three automakers in a bid to reach a deal.

holding the deal hostage

Ford President Jim Farley, who has remained relatively silent until now, came out strongly Friday against Fain's negotiating tactics.

"I believe we could have reached a compromise on pay and benefits, but so far the UAW is holding the deal hostage over battery plants," he told reporters.

Fain later hit back. "I don't know why Jim Farley is lying about the state of negotiations. It could be because he failed to show up for bargaining this week, as he has for most of the past ten weeks," he said.

"If he were there, he'd know we gave Ford a comprehensive proposal on Monday and still haven't heard back."

Ford recently paused construction at one of its plants and warned it may have to scale back its ambitions for the project.

Stellantis said in a statement it has been "intensely working with the UAW to find solutions to the issues that are of most concern to our employees while ensuring the company can remain competitive."

"We have made progress in our discussions, but gaps remain," it said, adding it was committed to reaching "a fair and responsible agreement that gets everyone back to work as soon as possible."

The strike gets political

The strike has ramped up the significance of Michigan's swing state status in the 2024 presidential election.

Earlier this week, Joe Biden became the first sitting US president to stand on a picket line, joining UAW members in Michigan in a bold pitch for blue-collar votes against likely election rival Donald Trump.

Wearing a UAW baseball cap, the Democrat used a bullhorn to tell red-shirted workers they deserved "a hell of a lot more than what you're getting."

Trump's Republican party is typically hostile toward unions, though is often able to pick up significant numbers of working-class votes. Trump spoke at a non-union auto facility in the Detroit suburbs saying that he wants "a future that protects American labor, not foreign labor."

On Friday, Fain called Biden's visit a "historic day."

"The most powerful man in the world shows up for one reason only," he said. "Because our solidarity is the most powerful force in the world."

"When we stand together united in the cause of economic and social justice, there's nothing we can't do," he added.