President Donald Trump’s White House violated federal law by withholding military aid to Ukraine, a top government watchdog announced on Thursday, dealing a blow to the administration’s case against impeachment as the Senate prepares to put Trump on trial.

The Government Accountability Office dismantled the White House Office of Management and Budget’s rationale for withholding the $214 million in congressionally appropriated security assistance, which Trump held up as he and his personal lawyer, Rudy Giuliani, pressed the Ukrainian government to reopen investigations into Trump’s political rivals.

Trump has consistently denied that withholding the aid was at all related to his and Giuliani’s efforts to dig up dirt on the youngest son of Democratic presidential candidate Joe Biden. That question is at the center of impeachment allegations referred to the Senate for trial this week.

“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 GAO concluded in a report released Thursday morning. “OMB withheld funds for a policy reason, which is not permitted under the Impoundment Control Act (ICA). The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA.”

“OMB’s assertions have no basis in law,” the report concluded.

The budget office disagreed with the findings. “We disagree with GAO's opinion,” the office said in a statement early Thursday. “OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the president’s priorities and with the law.”

For its part, the GAO said that “OMB and State have failed, as of yet, to provide the information we need”—and that “We consider a reluctance to provide a fulsome response to have constitutional significance.”

The report will fuel Democratic impeachment efforts right as the Senate trial gets underway. Trump’s critics have pointed to the withholding of aid to Ukraine as evidence that Trump sought to use the levers of U.S. foreign policy to advance his personal and political goals.

Trump and his allies, meanwhile, have insisted that the temporary hold placed on that aid was motivated purely by a concern over corruption in Ukraine, and a desire to properly steward U.S. taxpayer money.

The GAO report makes that case significantly more difficult.

“This bombshell legal opinion from the independent @USGAO demonstrates, without a doubt, that the Trump Administration illegally withheld assistance from Ukraine and the public evidence shows that the president himself ordered this illegal act,” tweeted Sen. Chris Van Hollen, a Maryland Democrat who requested the GAO investigation.

The federal law at issue provides only limited circumstances under which the White House can hold up foreign aid, including efforts to save taxpayer money or “to provide for contingencies.” But OMB didn’t invoke any of those specific provisions in either its initial justification for withholding the aid or in its communications with GAO investigators.

Instead, the agency said it was holding up the funds to prevent their use “in a manner that could conflict with the president’s foreign policy.” In other words, the concerns were of a policy nature. But “the ICA does not permit deferrals for policy reasons,” GAO concluded. “Thus, the deferral of [Ukraine aid] was improper under the ICA.”

The House officially transmitted its articles of impeachment to the Senate on Wednesday, but Nita Lowey, the Democratic chairwoman of the powerful House Appropriations Committee, signaled that Congress will seek to address the violations GAO found not just through impeachment, but also the normal legislative process.

“While Senate Republicans face a choice in the upcoming trial on whether to be loyal to the president or to the Constitution, Congress must also take legislative action to ensure effective management of taxpayer dollars going forward,” Lowey said in a statement on the GAO report. “Working with my colleagues on the Budget Committee, we will soon put forward strong reforms to address the Trump administration’s abuse of apportionment authority and appropriations law.”