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Biden fires back at Trump: 'I'm sick of this president badmouthing unions'

Former Vice President Joe BidenJoe BidenKaty Perry and her 'Firework' close out inauguration TV special Arizona Republican's brothers say he is 'at least partially to blame' for Capitol violence Tom Hanks: After years of 'troubling rancor,' Inauguration Day 'is about witnessing the permanence of our American ideal' MORE hit back at President TrumpDonald TrumpClinton, Bush, Obama reflect on peaceful transition of power on Biden's Inauguration Day Arizona Republican's brothers say he is 'at least partially to blame' for Capitol violence Biden reverses Trump's freeze on .4 billion in funds 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."

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.”

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“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!”

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 ClintonHillary Diane Rodham ClintonBiden's inauguration marked by conflict of hope and fear Schumer becomes new Senate majority leader Clinton says it meant 'great deal' to hold inauguration weeks after riot MORE.

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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.