The International Association of Fire Fighters (IAFF) has declined to endorse a candidate ahead of next month’s US presidential election, despite efforts by both the Kamala Harris and Donald Trump campaigns to court the union.

“This decision, which we took very seriously, is the best way to preserve and strengthen our unity,” the IAFF said in a statement.

The union, which has almost 350,000 members, was a key part of the coalition built by Joe Biden – and the first union to back the president’s run for election in 2020.

It is the second leading trade union to refrain from endorsing either Harris or Trump as tens of millions of Americans prepare to cast their votes. The Teamsters International, a US transportation workers union that represents more than 1.3 million workers, also announced it would not back a candidate.

Both campaigns had sought the IAFF’s support, with Tim Walz, Harris’s running mate, and JD Vance, Trump’s running mate, addressing the union’s convention in August.

Walz claimed in his speech that he had signed “the most comprehensive firefighter legislation in the nation” as governor of Minnesota. Vance, who grappled with boos from the audience, claimed that he and Trump represented a “new kind of Republican party” and would “never stop fighting” for first responders.

On Thursday, the IAFF said its executive board had voted by a margin of 1.2% to not endorse a presidential candidate. “We encourage our members – and all eligible voters – to get out and make their voices heard in the upcoming election,” said Edward Kelly, the union’s president.

It is not the first time the IAFF has refrained from backing a candidate. While it endorsed Barack Obama in 2008, it reportedly shelved plans to publicly support Hillary Clinton, the Democrat presidential candidate in 2016.