President Donald Trump on Wednesday morning retweeted dozens of people who said they were firefighters and that they supported him over former Vice President Joe Biden.

Biden on the 2020 campaign trail so far has taken to the strategy of relentlessly attacking Trump — prompting a social-media outburst from the president.

Other 2020 Democrats have criticized Trump as well, but Biden is seemingly making it his central campaign tactic.

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Former Vice President Joe Biden's campaign strategy thus far has been to go directly after Trump — perhaps more so than any other high-profile 2020 Democrat — and it appears to be getting under the president's skin.

This was evident on Wednesday morning as President Donald Trump went on a retweeting frenzy after the International Association of Firefighters, a key labor union, endorsed Biden.

In an early morning tweet on Wednesday, Trump said, "I've done more for Firefighters than this dues sucking union will ever do, and I get paid ZERO!"

Subsequently, Trump retweeted about 60 people who said they were firefighters and that they supported him over the former vice president.

Read more: Trump welcomes 'Sleepy Joe' Biden to the 2020 race by insulting his intelligence and bashing Democrats' 'very sick & demented ideas'

While Trump is usually freewheeling on Twitter, the cascade of retweets was a notable departure from his typical social-media style.

Indeed, Biden has been in the 2020 race for less than a week, and his tactic of taking Trump head-on has already produced a series of reactions from the president.

The escalating 2020 battle between Biden and Trump began the day the former vice president formally launched his campaign.

Biden jumped into the 2020 race on Thursday via a video that zeroed in on the August 2017 white nationalist rally in Charlottesville, Virginia, and Trump's controversial response to the deadly event.

Trump blamed "many sides" for the violence at the rally, which was attended by neo-Nazis and groups such as the Ku Klux Klan. The president also said there were "very fine people on both sides."

"With those words, the President of the United States assigned a moral equivalence between those spreading hate and those with the courage to stand against it. And in that moment, I knew the threat to this nation was unlike any I'd ever seen in my lifetime," Biden said in his announcement video.

Trump mocked Biden as "Sleepy Joe" in a tweet on the day of his campaign launch and doubled down on his response to Charlottesville while talking to reporters later that day.

Read more: We got the clearest sign yet that Joe Biden could dominate the 2020 Democratic field

The fight didn't stop there. Ahead of Biden's campaign-kick-off event in Pittsburgh on Monday, for example, Trump tweeted, "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!"

Biden followed up by ripping into the president at his rally.

"Donald Trump is the only president who has decided not to represent the entire country. We need a president who will work for all Americans," Biden said.

"Everybody knows who Donald Trump is," Biden added. "We have to choose hope over fear, unity over division and, maybe most importantly, truth over lies."

Biden also continued with his attacks on Trump's response to Charlottesville on Tuesday, referring to the president's characterization of the events as "nonsense."

Other top-ranked 2020 Democratic candidates, such as Sen. Bernie Sanders, have slammed Trump multiple times along the 2020 campaign trail but have largely prioritized focusing on policy.

But Biden's campaign so far has been typified by anti-Trump rhetoric, which has clearly triggered a full-throated response from the president. And the former vice president has taken notice.

At a rally in Iowa City, Iowa, on Wednesday, Biden said, "I understand the president's been tweeting a lot about me this morning. I wonder why the hell he's doing that? I'm going to be the object of his attention for a while."