By Susan Cornwell and Susan Heavey
WASHINGTON (Reuters) - The White House violated federal law by withholding security aid approved by lawmakers for Ukraine, a nonpartisan congressional watchdog said on Thursday, in a blow for U.S. President Donald Trump as the Senate prepared to hold a trial on whether to remove him from office.
The Democratic-led House of Representatives on Wednesday sent the Senate two charges it passed last month accusing Trump of abuse of power and obstruction of Congress arising from his dealings with Ukraine, clearing the way for only the third impeachment trial of a president to begin in earnest next week.
The abuse of power cited by the House included Trump's withholding of $391 million (£299 million) in security aid for Ukraine, a move Democrats have said was aimed at pressuring Kiev into investigating political rival Joe Biden, the Republican president's possible Democratic opponent in the Nov. 3 U.S. election.
"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 Government Accountability Office (GAO) concluded, referring to the fact that Congress had already voted to appropriate the funds.
An arm of Congress, the GAO is viewed as a top auditing agency for the federal government that advises lawmakers and various government entities on how taxpayer dollars are spent.
While the agency's assessment was a setback to Trump, it was unclear how or even if it would figure in his trial in the Republican-led Senate given that key issues such as whether witnesses will appear or new evidence will be considered remain up in the air.
Democrats said the GAO report showed the importance of the Senate hearing from witnesses and considering new documents in the trial.
"This reinforces - again - the need for documents and eyewitnesses in the Senate," House Speaker Nancy Pelosi told a news conference.
Senate Majority Leader Mitch McConnell, a Republican, has said senators should consider only the evidence amassed by the House.
The House voted on Wednesday 228-193, largely along party lines, to give the Senate the task of putting Trump on trial. The Senate is expected to acquit him, keeping Trump in office, as none of its 53 Republicans has voiced support for removing him, a step that requires a two-thirds majority.
Trump has denied wrongdoing and has called the impeachment process a sham.
DEMOCRAT SOUGHT REPORT
The GAO issued its opinion after receiving a letter inquiring about the aid from Democratic Senator Chris Van Hollen. The agency's findings are not legally binding, but its reports are seen by lawmakers as objective, reliable and generally uncontested. The GAO has no prosecutorial power.
Its report noted that the U.S. Constitution grants a president no unilateral authority to withhold funds in the way that Trump did. Instead, a president has a "strictly circumscribed authority" to withhold spending only in limited circumstances expressly provided by law. Holding up money for a policy reason, which the Trump administration did in this case, is not permitted, the report said.
Congress approved the $391 million to help Ukraine combat Russia-backed separatists in the eastern part of the country. The money ultimately was provided to Kiev in September after the controversy had spilled into public view.
A pivotal event leading to Trump's impeachment was a July 25 call in which he asked Ukraine's president to investigate Biden and his son Hunter Biden over unsubstantiated allegations of corruption and to look into a discredited theory that Ukraine, not Russia, interfered in the 2016 U.S. election.
Democrats have said Trump abused his power by asking a foreign government to interfere in a U.S. election for his own benefit at the expense of American national security.
Republicans have argued that Trump's actions did not rise to the level of impeachable offences. They have accused Democrats of using the Ukraine affair as a way to nullify Trump's 2016 election victory.
Ceremony, rather than substance, was expected to mark Thursday's proceedings in the Senate, with the seven House "managers" named to prosecute Trump to present the articles of impeachment to the Senate at noon (1700 GMT).
The Senate later in the day will invite U.S. Chief Justice John Roberts to the chamber to be sworn in to preside over the trial and, eventually, to swear in all 100 senators to serve as jurors. It will then formally notify the White House of Trump's impending trial.