One of America's most influential labour unions, the International Brotherhood of Teamsters, has declined to issue a US presidential endorsement for the first time since 1996.

The union, which boasts some 1.3 million members across the US and Canada, said it had received "few commitments on top Teamsters issues" from either Democratic nominee Kamala Harris or Republican nominee Donald Trump.

It also claimed that polling of its rank-and-file members found "no definitive support" for either candidate, though two of its recent polls indicated lopsided support for Trump.

The move is a major blow to the Harris campaign's efforts to win over working-class voters with less than 50 days before election night.

An endorsement had the potential to mobilise thousands of Teamsters who live, work and vote in the crucial battleground states of Pennsylvania, Michigan and Wisconsin.

Shortly after the announcement, some Teamsters regional councils representing more than half a million members in Michigan, Wisconsin, Nevada and California, said they would be endorsing Harris.

In a statement, the Harris campaign touted its support from "the overwhelming majority of organised labour" and noted that many Teamsters locals have gotten behind her candidacy.

"While Donald Trump says striking workers should be fired, Vice-President Harris has literally walked the picket line and stood strong with organized labour for her entire career," campaign spokesperson Lauren Hitt said.

The union's rank-and-file - a coalition of members that includes freight drivers, warehouse workers and airline pilots - has long been considered politically diverse.

General President Sean O'Brien has sought to build inroads with Republicans since he took over leadership of the executive board in 2022.

He has reached out to more populist figures within the party, such as US senators Josh Hawley of Missouri and JD Vance of Ohio, who is now Trump's running mate.

Mr O'Brien also met privately with Trump at his Mar-a-Lago estate in January, shortly before the former president attended a roundtable with the union's board at its headquarters in Washington DC.

Following that meeting, Trump said he believed he had a "good shot" at receiving the union's endorsement.

The board also met with President Joe Biden before he stepped aside as the Democratic nominee, as well as third-party candidates Robert F Kennedy Jr and Cornel West.

But the union alarmed Democrats when it made its first financial contribution to their opponents in years, donating $45,000 (£34,000) - the maximum allowed contribution - each to both the Democratic and Republican National Conventions in February.

Mr O'Brien also became the first Teamsters boss ever to address the Republican National Convention (RNC).

Invited to deliver a prime-time address at the event in Milwaukee, the union head praised Trump as "one tough SOB" but declined to endorse him.

He also later criticised Trump and top campaign surrogate and billionaire businessman Elon Musk over comments in which the two discussed firing workers who threaten to go on strike.

After his decision to speak at the RNC, Mr O'Brien did not receive an invitation from Democrats to address their party convention last month.

The party instead invited rank-and-file members to represent the organisation and to speak from the convention stage.

Some members of the Teamsters have expressed anger with Mr O'Brien over his right-wing outreach.

Last month, the Teamsters National Black Caucus and six union locals defied national leadership by endorsing Harris on their own and urging members to get behind her.

On Monday, Harris met with the Teamsters board in a long-delayed roundtable that lasted an hour and a half.

A New York Times report described the sit-down as "sometimes tense", but a Teamsters spokesperson disputed this characterisation when asked by the BBC.

During their meeting, the Times added, Harris told Teamsters leaders: “I’m confident I’m going to win this. I want your endorsement, but if I don’t get it, I will treat you exactly as if I had gotten your endorsement."

Speaking to reporters after Harris had made her pitch, Mr O'Brien noted that "there wasn’t a whole lot of difference" between the answers she and her predecessor, Biden, had provided.

Biden has routinely touted himself as "the most pro-labour president ever", pointing to policies that have made it easier for US workers to organise and that have prioritised union labour for federal government projects.

Last September, he made history as the first US president to walk a picket line, when he joined the United Autoworkers in Michigan in a strike against the Big Three US auto companies: General Motors, Ford and Stellantis.

The Biden administration also shored up the Teamsters pension fund to the tune of $36bn, which it says prevented cuts to the retirement incomes of over 600,000 members.

Mr O'Brien and other leaders have also acknowledged on several occasions that Biden has been "great for unions".

But before he dropped out in July, some reports suggested that the Teamsters did not plan to endorse Biden's bid for re-election.