Trump retweets some 60 anti-Biden tweets, elevating some questionable accounts It's the latest development in the confrontational Trump-Biden relationship.

President Donald Trump went on a spree Wednesday morning in attempt to show that he has support from firefighters after former Vice President Joe Biden, the newest 2020 Democratic presidential candidate, netted an endorsement from one of the nation's largest firefighter unions last week.

The International Association of Fire Fighters, a union that represents more than 300,000 firefighters, endorsed Biden last week. The endorsement was a loss for Trump and a sign that Biden may put up a strong fight for middle class voters who propelled Trump to his 2016 victory.

To push back on the endorsement, Trump retweeted about 60 Twitter users in less than an hour who said they supported him over Biden. Each of the users the president retweeted were responding to a tweet in support of Trump from a conservative radio host.

But it was unclear if each response from the nearly 60 strangers that were retweeted to Trump's 59.9 million followers were legitimate accounts or not -- a continuous question as another presidential election nears and the magnitude of social media disinformation grows.

Whom did the president retweet?

Of the users Trump retweeted Wednesday morning, one account clearly used "very carefully crafted verbatim content" directed at particular users and frequently used by coordinated disinformation campaigns, according to an analysis by Darren Linvill, an associate professor at Clemson University who researches social media and disinformation coordination, including the Russian Internet Research Agency.

Another user Trump retweeted was determined to likely be a bot, according to a tool developed by students at the University of California at Berkeley that tracks patterns of political bots.

A third user concerned Linvill for a different reason. Four days before her account was retweeted by the president, a Twitter user named Kimberly tweeted at Customs and Border Patrol that its agents should take two MS-13 gang members apprehended at the Southern border "outback" and "eliminate them for their own good."

"Even if they are not disinformation, even if each of these is an individual person, a lot of them are still really questionable as to why the president would elevate their perspective," Linvill said. "It's concerning to me that the president of the United States would retweet someone without having any knowledge of who they are because like it or not, a retweet can be perceived as an endorsement."

For Linvill and his team, the president's engagement with potential bots or accounts spreading disinformation is just a portion of a problem that is going to be "orders of magnitude worse than it was in 2016." For example, Linvill and his partner at Clemson, associate professor of economics Patrick Warren, recently flagged the most popular tweet from an IRA-run account that they'd seen to date, after analyzing roughly 3 million English language tweets known to be affiliated with the Russian agency. The tweet had 400,000 likes. On average, the tweets the president retweeted on Wednesday morning were liked by less than 10,000 users.

“It shows how organic coordination disinformation can appear and how really, really good they are. If they can fool educated, intelligent, media savy individuals then they can certainly fool my grandma, my children and me for that matter,” Linvill said.

On the road to 2020, Twitter is a frequent arena for Trump and Biden

At a campaign event in Iowa Wednesday afternoon, Biden said he'd been made aware of the tweets.

"I understand the president has been tweeting about me. Wonder what the hell that is about," Biden said. A supporter in the crowd yelled that it was because Trump "doesn't have anything else to do," prompting laughter from Biden.

Trump's comments on the firefighter's union endorsement Wednesday, when he said he had done more for firefighters than the "dues sucking union" and gets paid "ZERO," followed a back-and-forth with Biden on the endorsement last week.

The IAFF chose not to endorse a candidate for president in 2016 but endorsed former President Barack Obama in 2008 and 2012. A a union survey in 2016 showed support for Trump from IAFF union members.

IAFF president Harold Schaitberger stood behind the union's endorsement of Biden despite membership support for Trump in an interview with ABC News last week.

In 2016, “Biden wasn’t in the ballot," Schaitberger said.

"That’s a big distinction from then and now. He has 40 years of supporting firefighters in every single way, whether it is jobs money to protect their jobs during the Great Recession, or whether it's providing support for their families with the public officers death benefit bill," he said.

"He also has an accomplishment of leadership and experience," Schaitberger added. "This is about a track record of service and delivery for the American people and for workers – and those workers will be voting in the states that matter: Pennsylvania, Ohio, Wisconsin, Michigan. Joe Biden will connect."

Trump first went on the attack a few hours after Schaitberger announced the union's support for Biden, calling Schaitberger "the Dues Sucking firefighters leadership" and saying union support will always go to Democrats, "even though the membership wants me."

Biden responded to Trump's attack tweeting "I'm sick of this President badmouthing unions. Labor 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."

He added, "We need a president who honors them and their work."

ABC News' Devin Dwyer contributed to this report.