The Government Accountability Office (GAO) issued a legal opinion on Thursday saying that President Trump's administration broke the law by withholding defense aid to Ukraine — the issue at the heart of the president's impeachment trial.

That money, $214 million which had been allocated to the Department of Defense for security assistance, was appropriated by Congress and therefore the administration did not have the right to hold it back just because it disagreed with its allocation, the opinion from the nonpartisan government watchdog said.

"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," the opinion said. "[The Office of Management and Budget] OMB withheld funds for a policy reason ... not a programmatic delay. Therefore, we conclude that the OMB violated the ICA [Impoundment Control Act]."

The OMB made clear Thursday, however, that it disagreed with the GAO report.

“We disagree with GAO's opinion," OMB spokesperson Rachel Semmel said. "OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President's priorities and with the law."

Further, a senior administration official said to Fox News that they believed the OMB was trying to insert itself into impeachment at a time when media attention on the matter is high.

"GAO’s findings are a pretty clear overreach as they attempt to insert themselves into the media’s controversy of the day. Further, GAO has a history of the flip-flops, reversing 40-years of precedent this year on their pocket rescission decision, they were also forced to reverse a legally faulty opinion when they opposed the reimbursement of federal employee travel costs. In their rush to insert themselves in the impeachment narrative, maybe they’ll have to reverse their opinion again.”

Senate Minority Leader Chuck Schumer, D-N.Y., responded to the GAO's report Thursday morning with a statement re-upping his demand that the Senate consider additional documents as evidence in the upcoming impeachment trial.

“The GAO opinion makes clear that the documents we requested in our letter last month are even more needed now because GAO confirmed the president broke the law," Schumer said. “All senators will get a chance to vote to obtain these documents next week.”

The Trump administration, through the OMB, withheld a total of about $400 million of security assistance from Ukraine last summer. This came after Trump asked Ukrainian President Voldomyr Zelensky to investigate the family of his 2020 rival, Joe Biden, and while the White House allegedly was withholding an Oval Office visit from Zelensky in exchange for that investigation.

These actions are what fueled the impeachment effort against Trump, whose trial technically gets underway Thursday and moves into full swing next week.

The $214 million the GAO says was illegally withheld was allocated to the Department of Defense, while the rest was allocated to the State Department. The legal opinion said the GAO also had questions about the money from State, and that it was looking into whether that, "withholding was proper."

"The Constitution grants the President no unilateral authority to withhold funds from obligation," the opinion said. "Instead, Congress has vested the President with strictly circumscribed authority to impound, or withhold, budget authority only in limited circumstances as expressly provided in the ICA." Read more at FOX News