The International Brotherhood of Teamsters says the union will not endorse any US presidential candidate despite polling that showed a majority of members backed Republican candidate Donald Trump over Democrat Kamala Harris.
The 1.3 million-member union - which represents truck drivers and a wide range of other workers ranging from airline pilots to zookeepers - had released an electronic poll of its members earlier on Wednesday that showed rank-and-file members preferred Trump over Harris by 59.6 per cent to 34 per cent.
The Teamsters have endorsed every Democratic candidate for president since 2000 but have on occasion endorsed Republicans, including president Ronald Reagan in 1984 and vice president George HW Bush in 1988.
It is the first time since 1996 the union is not making an endorsement.
Most major US unions have endorsed Harris, including the United Auto Workers union.
A poll of rank-and-file members put Donald Trump over Kamala Harris by 59.6 per cent to 34 per cent. (AP PHOTO)
The AFL-CIO, which represents 60 unions and 12.5 million workers, endorsed Harris in July.
The Teamsters' executive board endorsement had been widely anticipated because it was seen as a factor in a handful of battleground states that will decide the November 5 election, including Michigan, Nevada and Pennsylvania, where union membership is strong.
"Neither major candidate was able to make serious commitments to our union to ensure the interests of working people are always put before Big Business," Teamsters President Sean O'Brien said in explaining the union's decision.
"We sought commitments from both Trump and Harris not to interfere in critical union campaigns or core Teamsters industries - and to honour our members' right to strike - but were unable to secure those pledges."
Harris campaign spokeswoman Lauren Hitt noted that despite the general union's decision, some Teamsters locals have endorsed Harris.
"When she is elected president, she will look out for the Teamsters rank-and-file no matter what - because they always have been and always will be the people she fights for," Hitt said.
The Trump campaign cited the Teamsters poll data saying it showed the "vast majority of rank-and-file working men and women in this important organisation want president Donald Trump back in the White House".
The union said that, following the September 10 presidential debate, independent polling firm Lake Research Partners found in a survey ending September 15 that Teamsters members selected Trump by 58 per cent for endorsement over 31 58 per cent for Harris.
O'Brien and other union representatives met with Harris on Monday after meeting with Trump in January.
The Teamsters said Teamsters' polling data showed that, before Biden's withdrawal from the presidential race on July 21, members backed Biden 44.3 58 per cent to Trump's 36.3 58 per cent.
O'Brien spoke to the Republican National Convention in July but also criticised Trump for suggesting that workers who go on strike could be fired.