Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellWhite House pleads with Senate GOP on emergency declaration Senate Dems seek to turn tables on GOP in climate change fight Pence meets with Senate GOP for 'robust' discussion on Trump declaration MORE (R-Ky.) is sending a clear warning to Donald Trump Donald John TrumpREAD: Cohen testimony alleges Trump knew Stone talked with WikiLeaks about DNC emails Trump urges North Korea to denuclearize ahead of summit Venezuela's Maduro says he fears 'bad' people around Trump 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."