WASHINGTON (Reuters) - President Joe Biden is on the brink of failing to win a key labor endorsement as leaders of the 1.3 million-member Teamsters union consider backing no candidate at all in the U.S. presidential race, according to two people familiar with the matter.

The International Brotherhood of Teamsters decision has not been finalized and is expected to be made in the coming weeks.

Not backing Biden, who the union endorsed in 2020, would compound political damage to the Democratic president's reelection bid.

A Teamsters endorsement for Republican candidate Donald Trump appears unlikely, sources say, but deep internal divisions mean the union may not back any candidate at all. That would mark the first time since 1996, according to news reports.

Since his halting performance in a presidential debate on June 27, Biden has already seen a number of lawmakers and donors ask him to stand aside, worried about his ability to get reelected and to serve another four-year term. Some allies say they believe Saturday's Trump assassination attempt could quiet those calls, but other Democrats doubt that.

Biden's team once viewed the Teamsters endorsement as all but inevitable, and still counts a number of senior leaders there as supporters. But months of deteriorating relations and rising concerns about Biden's political endurance have soured sentiment among some of the leaders at the union, which represents workers in fields ranging from trucking to manufacturing and office work.

"No final decision has been made," said Kara Deniz, a spokesperson for the Teamsters, adding that any reporting that suggests an outcome is speculative.

Last week, United Auto Workers President Shawn Fain met with that union's executive board to discuss his concerns over Biden's ability to defeat Trump.

Teamsters President Sean O'Brien will speak at the Republican National Convention in Milwaukee on Monday night but is not yet scheduled to speak at the Democratic convention in August.

O'Brien reached out simultaneously to the Democratic and Republican national committees to speak at their conventions but only heard back from Republicans, Deniz told Reuters.

A person familiar with the planning of the Democratic convention said no final decisions had been made about their programming.

"We are building a convention in Chicago that will tell our story to the American people, including the stories of labor and union leaders and workers that President Biden has been delivering for as the most pro-union president in modern history," said convention spokesperson Matt Hill.

BIDEN, CLINTON, OBAMA

The Teamsters, founded in 1903, endorsed Biden in 2020, as well as Democrats Hillary Clinton in 2016 and Barack Obama in 2008 and 2012, though they have sometimes picked Republicans in earlier elections.

The group's frustrations with Biden's team have mounted in recent months. On one key priority, rescuing trucking giant Yellow Corp and its 30,000 union jobs from bankruptcy, O'Brien sought and was denied an Oval Office meeting to discuss the issue with Biden, according to one of the people familiar with the matter.

The White House did not respond to a request for comment.

Those frustrations were compounded as some Democratic lawmakers and donors have since the debate called for Biden to drop his reelection bid and open the door for another candidate, another person familiar with the union's deliberations said.

Working-class groups helped power Biden's 2020 election victory in closely contested states such as Michigan, Pennsylvania and Nevada.

Union groups encourage their members to vote, volunteer and donate to campaigns, and they form a particularly important source of cash and labor for Democratic presidential campaigns.

In return, they expect policies that increase union jobs, wages and make it easier to collectively bargain contracts with employers. Biden's gestures have included a $36 billion bailout of a union pension fund that prevented cuts to the income of over 350,000 Teamsters union workers and retirees.

Across many unions, rank-and-file workers are more divided about Democrats than their leadership, and Trump has actively courted workers' support.

The Teamsters held roundtables with Trump and Biden this year and hosted some 300 local events with workers to gauge their opinions on the race.

Charles Lutvak, a Biden campaign spokesperson, said the campaign had drawn vast union support reflecting Biden's "record of delivering results for working families while Donald Trump delivers for his wealthy donors and himself."

He said Biden is "a champion for working people over greedy corporations - whether they vote for him or not."

O'Brien has said the union would conduct polling and likely make a decision on an endorsement after the parties' conventions have concluded next month.

(Reporting by Trevor Hunnicutt and Jarrett Renshaw; additional reporting by Nandita Bose; Editing by Heather Timmons and Deepa Babington)

By Trevor Hunnicutt and Jarrett Renshaw