Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE hit back at President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Monday shortly after Trump unleashed a string of Twitter attacks on the Democratic presidential candidate.

"I'm sick of this President badmouthing unions," tweeted Biden, who formally announced last week his intention to challenge Trump in 2020. "Labor built the middle class in this country. Minimum wage, overtime pay, the 40-hour week: they exist for all of us because unions fought for those rights. We need a President who honors them and their work."

I'm sick of this President badmouthing unions. Labor built the middle class in this country. Minimum wage, overtime pay, the 40-hour week: they exist for all of us because unions fought for those rights. We need a President who honors them and their work. — Joe Biden (@JoeBiden) April 29, 2019

Biden’s comment came hours after Trump accused labor leaders of breaking with union members by backing Democrats. He also claimed that union heads of routinely “rip-off their membership with ridiculously high dues, medical and other expenses while being paid a fortune.”

ADVERTISEMENT

“I’ll never get the support of Dues Crazy union leadership, those people who rip-off their membership with ridiculously high dues, medical and other expenses while being paid a fortune,” Trump wrote in one tweet. “But the members love Trump. They look at our record economy, tax & reg cuts, military etc. WIN!”

I’ll never get the support of Dues Crazy union leadership, those people who rip-off their membership with ridiculously high dues, medical and other expenses while being paid a fortune. But the members love Trump. They look at our record economy, tax & reg cuts, military etc. WIN! — Donald J. Trump (@realDonaldTrump) April 29, 2019

The back-and-forth between Trump and Biden came after the International Association of Firefighters (IAFF) announced its support for the former vice president’s nascent bid for the White House, the first major labor union to endorse in the 2020 presidential race.

The IAFF stayed out of the 2016 presidential election, declining to endorse Trump or Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE.

ADVERTISEMENT

While the endorsement was widely expected — Biden’s relationship with the IAFF goes back to the 1970s — it underscores how the former vice president is hoping to make support from labor unions a central part of his campaign.

Biden will appear in Pittsburgh on Monday to deliver the first major speech of his presidential bid. He’s expected to be joined by members of the United Steelworkers — another signal that Biden is leaning heavily on organized labor for early momentum.

Earlier this month, Biden appeared at a rally organized by United Food & Commercial Workers to support some 31,000 grocery workers striking over proposed changes to their wages and employment benefits.