Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE (R-Ky.) is sending a clear warning to Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, arguing the presumptive presidential nominee needs to pivot and act like a "serious candidate."

“It’s no secret that he’s struggling,” McConnell told Kickass Politics' Ben Mathis during a podcast posted Friday. “I’ve endorsed him because he’s earned the nomination, but he needs to start acting like a serious candidate for president if he wants to win the election.”

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The Senate Republican leader added that Trump has made "a number of very significant gaffes that have had him moving down in the polls."

Trump has spent weeks being chastised by lawmakers — including those in his own party — after he said a federal judge overseeing lawsuits against Trump University was biased because of his Mexican heritage.

Trump's campaign finished May with just $1.3 million in the bank. McConnell, referencing the campaign finance report, added, "It’s not much money in there."