Acting White House Chief of Staff Mick Mulvaney answers questions during a briefing at the White House October 17, 2019 in Washington, DC. Win McNamee | Getty Images News | Getty Images

But a policy reason "is not permitted under the Impoundment Control Act" to withhold funds appropriated by Congress, GAO said. "The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA." "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 report said. The agency noted that the Constitution "specifically vests Congress with the power of the purse." And the report said that Trump, like all other presidents, "is not vested with the power to ignore or amend any such duly enacted law." "Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law." House Speaker Nancy Pelosi, D-California, said, "The OMB, the White House, the administration, broke — I'm saying this —- broke the law." Tweet OMB spokeswoman Rachel Semmel, said, "We disagree with GAO's opinion." "OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President's priorities and with the law," Semmel said. A senior administration official, speaking on the condition of anonymity, said, "GAO's findings are a pretty clear overreach as they attempt to insert themselves into the media's controversy of the day." Trump's trial formally began in the Senate chamber later Thursday. But GAO General Counsel Thomas Armstrong said its report was not deliberately timed to release at the same time as the trial. "Our legal decisions are issued when we have completed all of our research and are ready to come to a sound conclusion," Armstrong said in a response to CNBC's inquiry about the timing of the release.