WASHINGTON (Reuters) - U.S. President Donald Trump fired off dozens of retweets from purported firefighters, their families and acquaintances on Wednesday, in his latest assault on Joe Biden, the newest Democratic candidate for the 2020 presidential nomination.

U.S. President Donald Trump arrives for an event honoring 2018 NASCAR Cup Series Champion Joey Logano at the White House in Washington, U.S., April 30, 2019. REUTERS/Joshua Roberts

Trump’s response comes two days after the country’s largest union of firefighters endorsed Biden, the former vice president who shot to the top of opinion polls after entering the race last week.

Trump began his tirade at dawn by quoting Fox News personality Dan Bongino, who said he knew no firefighters who would vote for Biden.

“I’ve done more for Firefighters than this dues sucking union will ever do, and I get paid ZERO!” Trump wrote on Twitter, without elaborating.

He followed that post by retweeting nearly 60 accounts who responded to Bongino by either criticizing Biden or professing support for Trump.

Biden did not respond to Wednesday’s tweetstorm but a campaign spokeswoman for Biden noted Trump’s disquiet with sarcasm.

“No, he doesn’t seem concerned at all,” Kate Bedingfield wrote on Twitter.

Though Trump, a Republican, has boasted he could handily beat any Democratic opponent, some of Biden’s supporters believe the former vice president may be able to capture votes from the same Midwestern states that steered Trump to victory in 2016.

Biden announced his candidacy on Thursday with a direct attack on Trump, whom he called a “threat to this nation.”

In his third bid for president, Biden has surged in the public opinion polls, with a CNN poll giving him a 15 percentage point lead in a field of 20 Democratic candidates.

Breaking with tradition, Trump has weighed in on a number of his would-be opponents, but reserved most of his barbs for Biden and U.S. Senator Bernie Sanders, another leading candidate.