President Donald Trump, still seething over a major firefighters union's endorsement of former Vice President Joe Biden, retweeted nearly five dozen tweets on Wednesday rebuffing Biden's claim to support from firefighters.

The 58 retweets before 6:30 a.m. ET sought to make the case that while the firefighters union's leadership supports Biden, many firefighters support Trump. While the retweets, which included a parody account, did not conclusively make that case, the spree belied Trump's public claims that he is not concerned about the prospect of facing Biden in the 2020 general election.

Biden responded during a campaign stop Wednesday in Iowa, joking that he expects to "be the object of his (Trump's) attention for a while."

"I understand the President's been tweeting about me this morning for a while. I wonder why he's doing that?" Biden said.

Earlier in the day, Biden's communications director tweeted of Trump: "No, he doesn't seem concerned at all." The Point: Donald Trump is doing a terrible job disguising his worries about facing Joe Biden in 2020

Privately, Trump has worried for weeks about Biden's appeal to voters in the Rust Belt states that helped deliver the President's 2016 victory. In the first week of Biden's candidacy, Trump has failed to keep those concerns private.

Trump's tweets also marked the latest instance of the President rejecting the advice he has gotten in recent weeks: to avoid taking any top Democratic candidate on one-on-one at the risk of boosting their chances of securing the Democratic nomination.

The President's retweets highlighted support from people who claimed to know or be firefighters who support Trump, not Biden, despite the International Association of Fire Fighters' endorsement of Biden.

"I've done more for Firefighters than this dues sucking union will ever do, and I get paid ZERO!" Trump claimed early Wednesday morning, before diving into the five dozen retweets.

One account, whose author claimed their husband was a longtime New York firefighter and would be voting for Trump in 2020, changed its name to “F*** Donald Trump” after the President retweeted it.

The author -- who replaced the account's avatar of a woman with a “Bernie 2020” logo -- then wrote, “What a great morning. In all seriousness, our president is an awful, hateful human being who wants nothing more than to keep the status quo and hatred prevalent in our country.”

A CNN poll conducted by SSRS after Biden's announcement last week cemented his status as the Democratic front-runner, with 39% of voters who are Democrats or Democratic-leaning independents saying he is their top choice for the nomination.

Biden has highlighted his longstanding support for trade unions and his appeal to blue-collar workers across the country.

“I am a union man,” Biden said during his first campaign rally, flanked by members of the firefighters' union. After Trump slammed union leaders last Friday, Biden hit back. “I'm sick of this President badmouthing unions,” Biden tweeted.

“Labour 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.”

Trump has appeared unnerved at Biden's candidacy and support from unions, and has launched attacks on the former vice president despite advice from some of his political advisers not to get sucked into a tit-for-tat with any of the top contenders for the Democratic nomination.

Several of the President's political advisers have warned him against targeting Biden and responding to Democratic candidates' attacks, wary that Trump might elevate a potential rival, sources have told CNN.

During meetings at the White House, Trump often asked about Biden's strength as a candidate as Biden itched closer to officially jumping into the race.

In his presidential campaign announcement and Tuesday's trip to Iowa, Biden lambasted Trump over his response to the 2017 white supremacist rally in Charlottesville, Virginia. (CNN)

Update news