WASHINGTON – Donald Trump really has a thing about Joe Biden.

Trump's aggressive campaign against the new Democratic presidential candidate triggered what may have been his busiest morning ever on Twitter on Wednesday.

More than 60 retweets in less than 20 minutes.

Apparently angry over Biden's endorsement by the International Association of Fire Fighters, Trump continually re-tweeted supporters who were critical of firefighter unions in general and Biden in particular.

"I’ve done more for Firefighters than this dues sucking union will ever do, and I get paid ZERO!" Trump himself said at the start of his massive retweet storm.

Trump then proceeded to re-tweet people who agreed with him.

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"This fire family does NOT support Joe Biden for President," said one critic.

Trump began the string by re-tweeting a comment from Fox News contributor Dan Bongino (who said "NONE of the Firemen I know are supporting Joe Biden for President.")

It appeared that he then went through the comments on Bongino's post and re-tweeted many of them.

Trump has maintained a steady barrage of attacks on Biden since the former vice president announced last week he was running for president – and many people see it as a sign of concern.

Ron Klain, a former chief of staff to Biden, said Trump knows that the former vice president would defeat him in a general election.

"Biden has clearly gotten under Trump’s skin," Klain said.

Biden himself, during an appearance later in the day in Iowa, mocked the president's social network activity.

“I understand the president’s been tweeting a lot about me this morning ... I wonder why the hell he’s doing that?" Biden said.

He added: “I’m going to be the object of his attention for a while.”