Van Hollen sent a letter on Monday to Gene Dodaro , the GAO comptroller general, outlining why he believes Trump's decision violated the Impoundment Control Act ( ICA ), which restricts a president's ability to withhold funds appropriated by Congress.

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"The Administration must be held accountable for its violations of the ICA , or we will open the floodgates for this and future Administrations to violate the ICA with impunity," Van Hollen continued.

The GAO announced last month that it was probing Trump's decision to delay $400 million in security assistance to Ukraine. The White House Office of Management and Budget (OMB) has said it followed the correct procedures in holding the funding, which was released in mid-September.

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Van Hollen said in his letter that under the ICA, a president has to notify Congress if they want to claw back or freeze funding already passed by lawmakers. But, "not only did the President not notify Congress of a deferral or rescission," Van Hollen noted that the Defense Department told Congress twice that the funds would be obligated.

"The Administration has failed to even state a legal reason under thefor its withholding of security assistance for Ukraine, and the evidence refutes the Administration’s stated reasons," Vanwrote in the letter, which was obtained by The Hill ahead of its release.Vanletter comes after the House voted to impeach Trump on two articles stemming from his decision to hold up Ukraine aid and his request that Ukrainian President Volodymyrhelp "look into" former Vice President Joeand his son Hunter. One article charged Trump with abusing power in his dealings with Ukraine and the second with obstructing Congress during its investigation of those actions.