STORY: Joe Biden may lose key union support... ahead of the U.S. presidential race, as leaders of the 1.3 million-member Teamsters union consider backing no candidate at all.

Sources say a final decision on the matter is expected to come in the next few weeks.

The union previously backed Biden in 2020... and working-class groups in closely contested states such as Michigan, Pennsylvania and Nevada helped power his election victory that year.

But people familiar with the union's deliberations point to months of deteriorating relations with Biden's team and some Teamsters leaders' concerns about Biden's political endurance.

Teamsters President Sean O'Brien also made an appearance at the Republican National Convention in Milwaukee on Monday night and gave praise to Biden's opponent, Republican candidate Donald Trump.

"But one thing is clear. President Trump is a candidate who is not afraid of hearing from new, loud and often critical voices, and I think we all can agree whether people like them or they don't like him in light of what happened to him on Saturday, he has proven to be one tough S-O-B (for son of a bitch)."

Still, sources say it's unlikely the Teamsters will endorse Trump either, due to deep divisions in the union.

It would be the first time since 1996 that the Teamsters considered backing no candidate at all.

The group's frustrations with Biden have grown in recent months.

O'Brien was denied an Oval Office meeting to discuss a key priority - rescuing trucking giant Yellow Corp and its 30,000 union jobs from bankruptcy.

A person familiar with the discussions say frustrations were compounded after calls from Democratic lawmakers and donors for Biden to drop his reelection bid following the presidential debate last month.