The US auto workers union expanded a strike against two of Detroit's "Big Three" on Friday, while President Joe Biden announced plans to join the picket line in solidarity with employees.

Some 5,600 members of the United Auto Workers union walked out of 38 US parts and distribution centers at General Motors and Stellantis at noon Friday, adding to last week's dramatic worker walkout.

The UAW has described its campaign as an effort to level the economic playing field for the working class, but Friday's events also underscored the lofty political stakes, with Biden's visit coming just a day before a planned trip by Republican rival Donald Trump.

In announcing the expanded strike, UAW President Shawn Fain said the latest action would stretch across 20 states, targeting sites owned by General Motors and Stellantis, with whom negotiations have stalled. 

"As we have said for weeks, we're not going to wait around forever for fair contracts at the Big Three," Fain said in a briefing.

Fain said Ford would be spared the latest strike escalation because of progress in talks at the company.

Fain extended an invitation to Biden to join workers on the picket line, saying the "way you can help is to build our movement and show the companies that the public stands with us."

By nightfall, the president announced his plan on X, the site formerly called Twitter.

"Tuesday, I'll go to Michigan to join the picket line and stand in solidarity with the men and women of UAW as they fight for a fair share of the value they helped create," Biden said. "It's time for a win-win agreement that keeps American auto manufacturing thriving with well-paid UAW jobs."

The visit will mark the US president's latest voyage to America's auto center. He has appeared at the 2022 Detroit Auto Show and at major project unveils alongside GM Chief Executive Mary Barra, Ford Chief Executive Jim Farley and other executives.

Biden's visit will come a day ahead of a plan by former president Trump to also visit Michigan as he seeks to score points with working-class voters.

- Ford gives ground -

Fain said Ford had improved earlier proposals by reinstating a cost-of-living measure that had been suspended in 2009.

The company also offered an enhanced profit-sharing system, the immediate conversion of temporary employees to full-time status, and granting the union the right to strike over plant closures.

"We're not done at Ford," Fain said, noting that the company was "serious about reaching a deal."

"At GM and Stellantis, it's a different story," he added.

Ford said in a statement that it "is working diligently with the UAW to reach a deal," adding that the two sides have made progress but "still have significant gaps to close on the key economic issues."

GM and Stellantis both pointed to an article in the Detroit News on leaked messages from the UAW's communications director discussing creating "operational chaos" at the automakers as a bargaining strategy.

GM said Friday that the UAW's latest move is "unnecessary," describing it as confirming the Detroit News report that "UAW leadership is manipulating the bargaining process for their own personal agendas."

"We question whether the union's leadership has ever had an interest in reaching an agreement in a timely manner," said a Stellantis statement that, as with GM, reiterated a commitment to bargaining in good faith to reach a deal.

Fain dismissed the Detroit News report, telling CNBC that the union's goal has always been to get a good agreement and that it needs to be "strategic" in how it pressures companies.

- Hitting consumers -

In targeting parts and distribution, the action could frustrate GM and Stellantis consumers who bring vehicles in for servicing.

The latest UAW stoppages "affect dealerships and consumers," said Alan Amici, president of the Center for Automotive Research, a nonprofit think tank in Ann Arbor, Michigan.

"It feels like the temperature has gone up a bit," Amici said of sharpening rhetoric on both sides.

The UAW's strategy of gradually expanding its action is part of what Fain has dubbed the "stand-up strike" -- an allusion to the union's landmark "sitdown" strike of the 1930s -- that aims to maximize the union's bargaining leverage due to the risk additional plants will be taken down.

The one-week old strike has so far had a limited effect on company profits, while introducing new pressures on auto parts suppliers to the Big Three. 

The three plants originally targeted produced midsized pickup trucks that are profitable, but not the biggest cash cows. 

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