The International Association of Fire Fighters (IAFF), a key supporter of President Joe Biden’s 2020 campaign, will not endorse Democratic presidential nominee Kamala Harris, becoming the second major union to do so in recent weeks.

The IAFF executive board voted by a margin of 1.2 percentage points not to endorse either Harris or former president Donald Trump, the union announced in a statement Thursday. “This decision, which we took very seriously, is the best way to preserve and strengthen our unity,” IAFF general president Edward Kelly said.

“The IAFF Executive Board determined that we are better able to advocate for our members and make progress on the issues that matter to them if we, as a union, are standing shoulder-to-shoulder.”

The IAFF endorsed a Democratic nominee in every presidential election since 1984, except in 2016 when it chose not to endorse Hillary Clinton or Trump. The IAFF was the first major union to back Biden’s presidential bid, Politico reported.

The announcement comes just two weeks after Harris became the first Democratic nominee in nearly 30 years not to receive an endorsement from the International Brotherhood of Teamsters. After internal polling showed 59.6 percent of members favored Trump and 34 percent backed Harris, the Teamsters executive board declined to endorse either candidate.