Washington (CNN) Acting White House chief of staff Mick Mulvaney was warned by a senior adviser last summer that he should "expect Congress to become unhinged" if the White House went through with withholding security aid to Ukraine, according to a previously undisclosed email conversation described by The New York Times.

"I'm just trying to tie up some loose ends," Mulvaney wrote in a June 27 email, according to the Times. "Did we ever find out about the money for Ukraine and whether we can hold it back?"

Robert Blair, the senior adviser to Mulvaney, replied over email that though withholding the aid back would be possible, "expect Congress to become unhinged" if the White House withheld those congressionally appropriated funds.

Blair also warned that withholding the aid could add to assertions that President Donald Trump was pro-Russia, the Times reported.

The question from Mulvaney came a week after the President initially asked about holding back the Ukraine assistance. Around the same time, Trump's personal lawyer, Rudy Giuliani, was pushing Ukraine to conduct internal investigations into former Vice President Joe Biden and his son, Hunter, over Hunter's work for a Ukrainian energy company, Burisma.

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