The Government Accountability Office said in an opinion Thursday that President Donald Trump violated federal budget law when he ordered White House officials to withhold most of a $250 million military aid package for Ukraine last summer.

The finding comes after House Democrats delivered articles of impeachment on abuse of power and obstruction of Congress grounds stemming from the Ukraine affair to the Senate Wednesday evening, triggering the Senate trial expected to start next week.

In the opinion, requested by Sen. Chris Van Hollen, D-Md., GAO said the withholding was an illegal “deferral” of money appropriated by Congress and ran afoul of a Nixon-era law aimed at ensuring funds are spent according to lawmakers’ intent.

[Ukraine aid legal ruling could shake up impeachment trial]

Van Hollen in a statement called it a “bombshell legal opinion” that “demonstrates, without a doubt, that the Trump Administration illegally withheld security assistance from Ukraine.” He charged that Trump “ordered this illegal act” and said the findings “reinforce the need for the Senate to obtain all relevant documents and hear from key fact witnesses in order to have a fair trial.”

In this case, the GAO focus was on the $250 million Ukraine Security Assistance Initiative funding for the Department of Defense appropriated for fiscal 2019. The agency said it was still reviewing a similar hold on $141.5 million in financing run through the State Department because officials said the White House budget office hadn’t provided enough information for a decision to be rendered.

Previously, White House budget office lawyers had characterized the funding hold as a “programmatic delay,” in which “operational factors unavoidably impede” the obligation of funds despite “reasonable and good faith efforts to implement the program,” using GAO definitions. 

[OMB: Ukraine aid delay was consistent with law, past practice]

The GAO opinion disagreed, arguing the administration withheld $214 million of the military assistance as a result of nine separate temporary holds.

“OMB violated the [1974 budget law] when it withheld DOD’s USAI funds from obligation for policy reasons,” the opinion said, referring to the Ukraine Security Assistance Initiative. “This impoundment of budget authority was not a programmatic delay.” 

“We disagree with GAO’s opinion,” Office of Management and Budget spokeswoman Rachel Semmel said in a statement. “OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President’s priorities and with the law.”

A senior administration official said GAO’s “findings are a pretty clear overreach as they attempt to insert themselves into the media’s controversy of the day.” The official said the congressional investigators have “a history of flip-flops” and added they may have to “reverse their opinion again” as further information comes to light.

[Ukrainian lives hung in balance as Trump held up aid]

OMB General Counsel Mark Paoletta said in a letter to executive branch agency general counsels last year that the executive branch is not bound by legal opinions from the legislative branch — the GAO is a legislative agency — because of the separation of powers doctrine.

The 1974 budget law allows the president to temporarily defer or withhold appropriated funds, but only for certain reasons and only if the president sends a special message to Congress explaining the reasons for the deferral.

GAO noted that Trump never sent such a message to Congress. The agency said Trump also violated the law because the hold did not comply with the required conditions for a deferral.

The funding hold was formally initiated July 25, the same day Trump spoke with Ukrainian President Volodymyr Zelenskiy on the phone to congratulate him on his recent election. Trump urged him to, among other things, open an investigation into former Vice President Joe Biden’s son Hunter Biden’s activities while serving on the board of a Ukrainian natural gas company.

While the letter freezing the funds was initially signed by Mark Sandy, a career OMB official, it was renewed several times by a political appointee, Michael Duffey, which budget veterans have said is highly unusual. Sandy testified in private before House investigators in November that two OMB officials resigned over the aid holdup, at least in part due to concerns about potential budget law violations.

Ultimately the funds were released on Sept. 12, but the Pentagon still couldn’t obligate $35.2 million of its $250 million fiscal 2019 Ukraine military assistance appropriation in time. Congress had to extend the funds’ availability in a stopgap funding law. The remaining $141.5 million in State Department foreign military financing funds were obligated on time.

GAO has reported periodic illegal impoundments over the years, including during the Trump administration and others during the presidencies of George Bush, Bill Clinton and George W. Bush. Typically, the funds were released for obligation, making further action unnecessary.