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 lashed out at 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, ahead of the Democratic candidate hitting the campaign trail for the first time since entering the 2020 presidential race.

In a string of tweets, Trump vented his frustration that Biden scored the endorsement of a major firefighters union, knocked him for staging his first campaign rally in Pennsylvania and slammed the news media for focusing on a candidate Trump sees as a relic of the past.

The Media (Fake News) is pushing Sleepy Joe hard. 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! — Donald J. Trump (@realDonaldTrump) April 29, 2019

The flurry of attacks suggest Trump views Biden as a formidable general-election opponent who could gain support from white, working-class voters in states Trump won in 2016, including Pennsylvania, Michigan and Wisconsin.

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Rank-and-file union workers “love Trump,” the president argued, because of his administration's record on the economy. But he said “Dues Crazy union leadership” who “rip-off their membership with ridiculously high dues, medical and other expenses” throw their support behind Democrats.

“I'm sick of this President badmouthing unions,” Biden responded later in a tweet. “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 on Monday won the backing of the International Association of Fire Fighters (IAFF), the first major labor group to endorse a 2020 candidate, giving him a boost as he prepares to stage his opening campaign rally in Pittsburgh later in the day.

“The Dues Sucking firefighters leadership will always support Democrats, even though the membership wants me. Some things never change!” Trump tweeted.

The IAFF did not endorse Democratic presidential 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 in 2016, abandoning their past practice of supporting the party’s presidential nominees. The decision was seen as a harbinger of Democrats’ struggles with white, working-class voters that helped propel Trump to the White House.

As Biden prepared to travel to the Keystone State, Trump suggested the economy would help keep the state in the Republican column next year.

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“Sleepy Joe Biden is having his first rally in the Great State of Pennsylvania. He obviously doesn’t know that Pennsylvania is having one of the best economic years in its history, with lowest unemployment EVER, a now thriving Steel Industry (that was dead) & great future!” Trump tweeted.

But there is evidence Trump may struggle to duplicate his success there in 2020.

Three in five registered voters in Pennsylvania said it is time for a change in the White House, while 1 in 3 said Trump has done a good enough job to win reelection, according to a Franklin & Marshall College poll released last month.

Other polls indicate Biden would be Trump’s toughest general-election opponent if he can win his party’s nomination. The latest Hill-HarrisX survey found Biden leading Trump by 6 percentage points among registered voters nationwide who were polled.

Trump has repeatedly criticized Biden in an apparent effort to weaken his standing in the crowded Democratic primary field.

The 72-year-old president last week described himself as a “young, vibrant man,” especially compared to Biden, 76.

“I look at Joe, I don't know about him,” Trump told a reporter who asked if age is a factor in the race. “I would never say anyone is too old. But they are all making me look very young, both in terms of age and in terms of energy.”

He also questioned Biden’s intelligence in an interview Thursday with Fox News host Sean Hannity Sean Patrick HannitySunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Cruz: Trump should nominate a Supreme Court justice next week Ex-Pence aide: Trump spent 45 minutes of task force meeting 'going off on Tucker Carlson' instead of talking coronavirus MORE, saying he refers to the former vice president as “Sleepy Joe” because “I’ve known him for a while and he’s a pretty sleepy guy.”

Trump added that Biden would not have the capacity to deal with Chinese President Xi Jinping.

“That's a different level of energy and, frankly, intelligence,” Trump said.

Biden took direct aim at Trump last week when he launched his presidential campaign with a video criticizing the president’s response to the deadly 2017 white supremacist march in Charlottesville, Va.

“If we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation — who we are — and I cannot stand by and watch that happen,” Biden said in the video.

—Updated at 12:25 p.m.