Trump, whose narrow win in Pennsylvania was one of the keys to his upset of Democrat Hillary Clinton in 2016, lashed out at the IAFF's endorsement of Biden.
Former US vice president Joe Biden took his bid for the Democratic presidential nomination to union workers in Pennsylvania on Monday as he and Republican Donald Trump vie for the support of organized labor in the crucial eastern state.
In his highly anticipated address, ex-president Barack Obama's number two spoke for less than 30 minutes -- much shorter than Trump's speeches to his base -- to several hundred union members.
"I make no apologies. I am a union man. Period," the 76-year-old Biden said after winning the endorsement of the International Association of Fire Fighters (IAFF).
"If I'm going to be able to beat Donald Trump in 2020 it's going to happen here," he told the crowd in Pittsburgh, a blue-collar city now remaking itself as a tech hub.
"With your help we're going to be able to do that," Biden said at his first campaign rally since he announced last week that he was joining the crowded field of 2020 Democratic nominee hopefuls.
In this city marked by the worst anti-Semitic attack in US history six months ago, Biden also shared his sorrow about another attack at a California synagogue last weekend.
"We're reminded again that we are in a battle for America's soul," he said.
Organized labor has long been a pillar of Democratic Party support, but Trump drew significant backing from white working-class voters -- particularly men -- in the 2016 election, and is hoping to do so again in 2020.
Trump, whose narrow win in Pennsylvania was one of the keys to his upset of Democrat Hillary Clinton in 2016, lashed out at the IAFF's endorsement of Biden and claimed that economic progress in the state would earn him the support of voters there.
"The Dues Sucking firefighters leadership will always support Democrats, even though the membership wants me," Trump tweeted. "Some things never change."
Trump accused the media of "pushing Sleepy Joe hard" and suggested Biden's record was partly responsible for the president's own rise to power.
"Funny, I'm only here because of Biden & Obama. They didn't do the job and now you have Trump, who is getting it done - big time!" he tweeted.
'Battle for America's soul'
Biden has set himself apart from most of the sprawling Democratic field by opting for a strategy of full-on confrontation with Trump.
During his speech, he logged several verbal volleys at the president, saying "we are in a battle for America's soul."
"Donald Trump is the only president who decided not to represent the whole country," said the longtime senator from Delaware, who broke into a large smile as he spoke behind a wooden podium while wearing a pale blue shirt. "We need a president who works for all Americans.
Americans need to "choose hope over fear, unity over division and maybe, most importantly, truth over lies," he said.
Addressing the crowd of firefighters, teachers and other union members, who broke out repeatedly into chants of "We want Joe!" Biden said: "The country wasn't built by Wall Street bankers, CEOs and hedge fund managers.
"It was built by you!"
Biden said the minimum wage should be raised to $15 an hour and listed affordable education, quality health care and clean renewable energy as his priorities.
In a Democratic Party that is increasingly leaning to the left, Trump boasts of adopting a more centrist and moderate approach, one that appeals to voters like Samantha Patrick.
"We really feel like he's the candidate who's going to be able to be Trump and just definitely wanted to be here to support him for the kickoff of his campaign," the 32-year-old nurse told AFP.
She was first in the line in the morning to enter the event with two children.
"I'm a little bit more on the liberal side, but I think moderation is the key at this point," said Rick Astle, 42, who manages a small business in downtown Pittsburgh.
"If we're going to heal this nation and the divide that we have, it's going to take coming to the middle and somebody that works across the aisle and I think he has a good track record, he's progressive."
A Washington Post/ABC poll on Sunday put Biden in the lead among Democrats, with 17 percent support, to 11 percent for Senator Bernie Sanders and five percent for South Bend, Indiana, Mayor Pete Buttigieg.
He raised $6.3 million in the 24 hours after his announcement, the highest figure for any Democrat so far.
Pennsylvania voted for Trump in 2016 -- after favoring Obama in the two previous elections -- making it one of the key industrial states where a sense of social and economic decline and alienation seemed to play into Trump's hands.
But Biden has long been a favorite of blue-collar voters, and he prides himself on staying close to the Democrats' working-class supporters.
After Pittsburgh, Biden will start campaigning at a more frenetic rhythm, traveling Tuesday and Wednesday in Iowa, the first state that will vote in Democratic primaries and caucuses in February. He returns to Pennsylvania for a rally on May 18 in Philadelphia.