Democratic Senator Chris Van Hollen is asking Congress' watchdog office to answer whether the administration's suspension of Ukraine aid violated federal law, as the articles of impeachment passed by the House remain in limbo.

Van Hollen wrote a letter to Government Accountability Office (GAO) Comptroller General Gene Dodaro on Monday, asking GAO to provide an opinion on the legality of withholding federal assistance to Ukraine under the Impoundment Control Act (ICA). Under the law, the executive branch is required to commit funds appropriated by Congress, although the law does allow the executive branch to withhold funds under limited circumstances.

GAO has already said it intended to conduct a legal analysis of the aid withholding, so Van Hollen is asking the congressional watchdog to provide that conclusion.

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"The House Permanent Select Committee on Intelligence (HPSCI) has concluded — and I agree — that the facts lead to the inescapable conclusion that the administration illegally impounded the funds for Ukraine," Van Hollen wrote. "Throughout the time that the executive branch withheld funds for Ukraine, numerous officials repeatedly raised concerns about violating the ICA. The evidence we have seen to date shows that the president of the United States abused his power and betrayed the trust of the American people by pressuring Ukraine to intervene in the 2020 election on his behalf."

Van Hollen pointed to the memorable press conference by acting White House chief of staff Mick Mulvaney and testimony that officials within the Office of Management and Budget were unaware of the reason for the hold, among other things, to argue that the halt in aid to Ukraine violated the law. The Maryland senator suggests the aid holdup was not merely programmatic in nature, but a direction from the president for political reasons.

"The administration has failed to even state a legal reason under the ICA for its withholding of security assistance for Ukraine, and the evidence refutes the administration's stated reasons," Van Hollen writes. "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."

Meanwhile, the Washington Post and New York Times reported last week that Trump administration officials had threatened a presidential veto if House Democrats did not drop language in a spending bill requiring the swift release of military aid for Ukraine in the future.

Mr. Trump, in response to last week's impeachment over the Ukraine matter, has been tweeting and retweeting supporters' defenses of him. But it's unclear when his Senate trial will take place, with the articles still residing in the House. House Speaker Nancy Pelosi says she doesn't want to appoint impeachment managers until she knows what kind of trial will take place in the Senate. Senate Majority Leader Mitch McConnell on Monday said lawmakers will be revisiting the issue after the holidays.