By David Shepardson

WASHINGTON More than half of the Senate's Democrats are backing the United Auto Workers' push for higher wages and benefits for workers at Detroit Three automakers' joint venture battery plants, they said in a letter released on Friday.

The 28 senators said the workers are critical to the United States reaching clean vehicle goals, offering the union a boost as it works to secure new contracts before its four-year deals with General Motors, Ford Motor and Chrysler-parent Stellantis expire Sept. 14.

UAW leaders have sought to secure support from Washington as negotiations opened earlier this month, with UAW President Shawn Fain meeting with lawmakers and President Joe Biden last week.

In their letter to the CEOs of the Detroit Three and battery joint ventures, Senate Majority Leader Chuck Schumer joined Senators Sherrod Brown, Ron Wyden, Bernie Sanders, Richard Durbin and others in urging the automakers to embrace the joint venture battery workers before the current contract expires.

"Workers in the new electric vehicle sector will be critical to your future success," the senators said, noting federal support for EVs in last year's sweeping Inflation Reduction Act.

It is a "national disgrace that the starting wage at any current American joint venture electric vehicle battery facility is $16 an hour," they wrote in the letter dated on Thursday, citing "poverty-level wages" amid "extreme financial gains for the companies, executives, and investors."

GM declined comment. Representatives for SK On, a unit of South Korea's SK Innovation, and Ford did not respond to a request for comment.

Stellantis said it respects the UAW's right to organize future hourly employees at its joint venture battery facilities, adding: " The joint venture intends to offer very competitive wages and benefits."

Ford CEO Jim Farley and executive chairman Bill Ford also met with lawmakers this week to discuss the UAW talks and other topics, sources told Reuters.

Biden, whose administration has been key in other recent labor negotiations, has described himself as the nation's most pro-union president. The White House last month said the president shared UAW's goal of good-paying jobs.

The UAW has not yet endorsed Biden's reelection as it waits to see how the shift to electric vehicles plays out, saying it wants a fair transition.

Fain on Friday said the "senators agree that now is the time to ensure all autoworkers have the same pay and safety standards that generations of UAW members have fought for and maintained."

Last month, the union chief criticized $9.2 billion federal loan to a Ford/South Korea's SK On joint venture. He has also criticized a GM/LG Energy Solution battery plant in Ohio over worker pay.

(Reporting by David Shepardson; Editing by Susan Heavey)