The big picture: The GAO found that the OMB violated the 1974 Impoundment Control Act, which requires that the White House and its agencies disburse funds appropriated by Congress as directed by the legislative branch.
The Office of Management and Budget placed a hold on the aid on July 25, "pending a policy decision", and was lifted on September 12, the office's general counsel Mark Paoletta has said. The withholding was not a programmatic delay.
The House impeachment inquiry concluded that Trump's administration improperly withheld the aid as leverage in an effort to get Ukraine to open investigations that would help the president politically.
Fox News' Kevin Corke reports on the latest news on the Trump impeachment battle.
"This bombshell legal opinion from the independent Government Accountability Office demonstrates, without a doubt, that the Trump Administration illegally withheld security assistance from Ukraine". Chris Van Hollen, a Maryland Democrat who asked for the GAO's evaluation. The Senate is set to begin its trial on Thursday. "OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President's priorities and with the law", said OMB spokeswoman Rachel Semmel.
Past presidents have also been dinged by the GAO for illegal behavior in shifting money, though more often it's for spending money they don't have, rather than not spending money they do have. But the GAO rejected that argument. "OMB's assertions have no basis in law".
"Congress makes funding decisions, and the Trump Administration's illegal impoundment of these vital national security funds was a brazen assault on the checks and balances inherent to our democracy", said House Appropriations Committee Chairwoman Nita Lowey, D-N.Y.
I don't doubt that many of Trump's Republican allies will shrug off the latest evidence of presidential wrongdoing, but their indifference to his illegalities is suddenly even more hard to defend.