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(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.

In the wake of an intelligence community whistleblower report expressing concerns the White House’s interactions with Ukraine, which would eventually serve as a basis for the impeachment inquiry, the White House released the Ukraine aid. The Democrat-led inquiry and Trump’s eventual impeachment established how Trump was involved in efforts to withhold the aid despite pushback from federal agencies and described the political pressure to Ukraine to conduct politically advantageous investigations.

During a White House press conference in October, Mulvaney said Trump held up an aid package to Ukraine in exchange for an investigation into an unsubstantiated theory that Ukraine, not Russia, was responsible for hacking Democratic Party emails in 2016. But he later attempted to retract that assertion.

Senate Minority Leader Chuck Schumer has called for Mulvaney to testify before the Senate.

Though Trump has been impeached by the House of Representatives, House Speaker Nancy Pelosi has not yet delivered the Articles of Impeachment for a Senate trial.

CNN’s Devan Cole contributed to this report.