WASHINGTON — President Trump unleashed a barrage of retweets early Wednesday that appeared to counter a notion that Democratic presidential hopeful Joseph R. Biden Jr. has the backing of all the firefighters in the country. The sheer volume of retweets signaled the president’s anxiety about losing union support in the 2020 presidential election to Mr. Biden, who now leads the crowded Democratic pool of candidates.

The national firefighters’ union endorsed Mr. Biden, the former vice president, on Monday, just days after he announced his campaign to run for the Democratic nomination. During his first campaign speech the same day, Mr. Biden positioned himself as the best president for union workers, a group of voters who supported Mr. Trump more than expected in the 2016 election.

“I make no apologies. I am a union man. Period,” Mr. Biden said in his speech.

Mr. Trump presented about 60 retweets on Wednesday from Twitter users as evidence to the contrary.

“I’ve done more for Firefighters than this dues sucking union will ever do, and I get paid ZERO!” Mr. Trump wrote in one Twitter post between dozens of retweets.