President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE during a campaign rally Friday repeatedly singled out former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE as he continues to rail against one of his top political rivals.

The former vice president, who has emerged as a frontrunner in the crowded 2020 Democratic primary field, is a favorite foil of the president, who often hits Biden on his age despite the fact that the Democrat is just three years older than him.

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“We’ve now named him, very slow ‘Sleepy Joe.’ He’s gotten slower and slower," Trump said to cheers from the crowd in Mississippi. "I’m afraid if he gets the nomination he’ll be so slow we’ll have the lowest-rated debates in history."

“I don’t think Sleepy Joe will get it,” he added, referring to the Democratic 2020 nomination.

Biden has emerged as a particularly popular target for Trump in recent days as the president seeks to defend himself against the House’s impeachment investigation over his dealings with Ukraine.

While the House investigates claims that Trump tied $400 million in aid to Ukraine in exchange for Kiev announcing an investigation into Biden and his son’s business dealings in the country, the president has turned to repeatedly floating unsupported claims that Biden abused his power as vice president to help his son.

“You know in a certain way, I was impressed because I didn’t think he had the energy, the brain power,” Trump said.

Some observers have hinted that Trump’s repeated bashing of Biden could indicate a concern from the his campaign that the former vice president could pose a serious challenge next year, particularly among white working class workers who traditionally vote Democrat but backed Trump in 2016.

While polls show a competitive race in the Democratic primary contest, several national and swing state surveys show Biden besting the president in a hypothetical 2020 matchup.