Earlier this month, a court-appointed monitor said Fain received 69,118 votes and Curry has 68,473 votes - a difference of 645 votes - but with 1,608 unresolved challenged ballots. The court monitor oversaw counting on Thursday but has not released results.

A group backing Fain said on Friday he has a lead of 505 votes with 600 challenged ballots left to be counted, "making Fain's victory all but assured." The monitor did not immediately comment on Friday.

The protest filed by Curry's campaign raised numerous issues including the fact that tens of thousands of ballots were returned as undeliverable and urged the monitor to "immediately delay the announcement of a final result and immediately investigate these questions in a transparent process."

The election comes at a critical time for the union.

Labor contracts with Detroit's Big Three automakers expire in September. The UAW is working to organize new battery plants and members worry that shifting to electric vehicles will cost jobs.

Curry has been president of the UAW since June 2021, and a UAW member since 1992. Fain has been a UAW member for more than two decades, serving as an officer at a local in Indiana representing workers at a Stellantis NV casting plant.

The UAW won a victory in December, when workers at an Ohio General Motors-LG Energy battery cell factory voted to join the union.

UAW officers previously were elected through a delegate system. Members approved direct elections in a 2021 referendum required as part of a 2020 Justice Department settlement to resolve a corruption probe which resulted in the incarceration of two former UAW presidents.

The UAW has about 375,000 U.S. members, down from 1.5 million in 1979.

(Reporting by David Shepardson in Washington; Editing by Matthew Lewis)

By David Shepardson