The International Brotherhood of Teamsters, one of the largest and most influential US labour unions, has declined to make a presidential endorsement, breaking with a quarter-century tradition of backing Democratic White House hopefuls.

"Unfortunately, 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 general president Sean O'Brien said in a statement on Wednesday.

The move is a blow to US Vice President Kamala Harris, this year's Democratic nominee who has positioned hers elf as a fierce defender of American labour. In the Teamsters' internal survey conducted in July, Biden held a 44-36 percent lead over Donald Trump. Today, Trump has surged ahead, commanding a 60-34 lead over Kamala Harris in a Teamster online and phone poll.

Blue and white collar workers

The Teamsters have previously backed GOP presidential candidates like Richard Nixon, Ronald Reagan, and George H W Bush. O’Brien's recent engagement with former President Trump and other Republicans had already fuelled speculation that the union might once again diverge from its usual alliances.

O'Brien has been actively forging connections with Republicans, including Senator Josh Hawley (R-Mo), to push forward the union's key policy goals, such as the Protecting Workers’ Right to Organise (PRO) Act, which enhances workers' ability to unionise and negotiate, as well as resisting national right-to-work legislation.

Founded in 1903 through the merger of the Team Drivers International Union and the Teamsters National Union, the union currently represents a diverse range of blue and white collar workers in both public and private sectors, with approximately 1.3 million members as of 2015.