Washington (CNN) President Donald Trump appeared unnerved as Joe Biden secured the endorsement of the top firefighters' union, issuing Monday morning a string of four tweets targeting the Democratic presidential candidate and union leaders.

That's exactly the behavior several of the President's political advisers have been warning him against, wary that Trump will elevate a potentially formidable rival, two sources familiar with the discussions told CNN.

"Sleepy Joe Biden is having his first rally in the Great State of Pennsylvania. He obviously doesn't know that Pennsylvania is having one of the best economic years in its history," Trump tweeted. "The Dues Sucking firefighters leadership will always support Democrats, even though the membership wants me. Some things never change!"

Within hours, the former vice president fired back, showing just why Trump's advisers might be concerned.

"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. We need a President who honors them and their work," Biden tweeted, engaging in the mano a mano Twitter fight many of his rivals would relish.

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