House impeachment managers (from left) Adam Schiff, Jerrold Nadler, Zoe Lofgren, Hakeem Jeffries and Jason Crow arriving for the start of the Senate impeachment trial in the US Capitol in Washington, D.C., January 16, 2020.

As the US Senate formally opened the impeachment trial on whether to remove Donald Trump from office, a nonpartisan congressional watchdog on Thursday dealt the Republican president a blow by concluding that the White House violated the law by withholding security aid approved for Ukraine by US lawmakers.

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Democrat Adam Schiff, who heads a team of seven House of Representatives members who will serve as prosecutors, appeared on the Senate floor to read the two charges passed by the House on December 18 accusing Trump of abuse of power and obstruction of Congress arising from his dealings with Ukraine.

The trial's opening formalities continued, with US Chief Justice John Roberts sworn in to preside over the proceedings. Roberts then swore in all 99 senators who were present to serve as jurors.

After the swearings-in, the Senate adjourned until 1:00 pm (1800 GMT) on Tuesday, when the prosecution begins laying out its case against the president in only the third impeachment trial in US history.

The abuse of power cited by the House included Trump's withholding of $391 million (€351m) in security aid for Ukraine, a move Democrats have said was aimed at pressuring Kiev into investigating political rival Joe Biden, the president's possible opponent in the 2020 US general election.

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"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 ridiculed the impeachment process for months, and he responded to the opening of the trial by once more branding it a "hoax".

"I think it should go very quickly," Trump told reporters in the Oval Office.

"It's totally partisan,” Trump said. "I've got to go through a hoax, a phony hoax put out by the Democrats so they can try and win an election."

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Democrat sought report

The GAO issued its opinion after receiving a letter enquiring 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 US 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.

Asked about the GAO report, House Republican leader Kevin McCarthy defended Trump's withholding of aid, citing concerns about corruption in Ukraine's new government.

"I think it was the rightful thing to do," McCarthy told a news conference.

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 December 18 impeachment by the House and his coming trial in the Senate 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 US election.

I JUST GOT IMPEACHED FOR MAKING A PERFECT PHONE CALL! — Donald J. Trump (@realDonaldTrump) January 16, 2020

Schiff indicated that the House prosecutors were considering calling Lev Parnas, an associate of Trump's personal lawyer Rudy Giuliani, as a witness if the Senate permits testimony in the trial.

"We are continuing to review his (media) interviews and the materials he has provided to evaluate his potential testimony in the Senate trial," Schiff said in a statement.

Giuliani has said Parnas, a Ukraine-born US citizen, helped him in investigating the Bidens. Documents released this week indicate Parnas was also involved in monitoring the movements of former ambassador to Ukraine Marie Yovanovitch before Trump removed her in May after being urged to do so by Giuliani.

In several stunning media interviews this week, Parnas said the US president was fully aware of efforts to pressure Ukraine to investigate Biden.

The 47-year-old Florida businessman told MSNBC he delivered an ultimatum in May to Volodymyr Zelensky, the incoming president of Ukraine, that no senior US officials would attend his inauguration and all American aid to the war-torn country would be withheld if an investigation into Biden wasn’t announced. The day after Parnas said he delivered the message, the State Department announced that Vice President Mike Pence would no longer be attending Zelensky’s inauguration.

Parnas said every communication he had with Zelensky's team was at the direction of Giuliani, whom he regularly overheard briefing Trump about their progress by phone.

“President Trump knew exactly what was going on," said Parnas, who is himself facing a raft of criminal charges related to campaign finance violations. "He was aware of all my movements. I wouldn't do anything without the consent of Rudy Giuliani, or the President,” he told MSNBC’s Rachel Maddow.

"It wasn't just military aid. It was all aid," Parnas tells @maddow. "Basically the relationships would be sour. We would stop giving them any kind of aid, unless... there were several demands at the point — the most important one was the announcement of the Biden investigation." pic.twitter.com/DYwDUofVnl — MSNBC (@MSNBC) January 16, 2020

Democrats have said Trump abused his power by asking a foreign government to interfere in a US 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 offenses. They have accused Democrats of using the Ukraine affair as a way to nullify Trump's 2016 election victory.

The Senate will formally notify the White House of Trump's impending trial later on Thursday.

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(FRANCE 24 with REUTERS, AP and AFP)

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