The trial, only the third such undertaking in American history, is unfolding at the start of the election year, a time of deep political division in the nation.

Senators filled the chamber, for a trial that will test not only Trump's presidency but also the nation's three branches of power and its system of checks and balances.

The US chief justice John Roberts is the presiding officer. He has long insisted judges are not politicians and is expected to serve as a referee for the proceedings. .

Trump faces two charges after the House voted to impeach him last month. One, that he abused his presidential power by pressuring Ukraine to investigate Democratic rival Joe Biden, using military aid to the country as leverage. Trump is also charged with obstructing Congress' ensuing probe.

The president insists he did nothing wrong, and dismissed the trial anew: day "It's totally partisan. It's a hoax."

Eventual acquittal is expected in the Republican-controlled Senate. However, new revelations are mounting about Trump's actions toward Ukraine.

The Government Accountability Office said Thursday that the White House violated federal law in withholding security assistance to Ukraine, which shares a border with hostile Russia.

At the same time, an indicted associate of Trump's personal lawyer Rudy Giuliani, Lev Parnas, has turned over to prosecutors new documents linking the president to the shadow foreign policy being run by Giuliani.

The developments applied fresh pressure to senators to call more witnesses for the trial, a main bone of contention that is still to be resolved. The White House has instructed officials not to comply with subpoenas from Congress requesting witnesses or other information.

House Speaker Nancy Pelosi said the new information from Parnas demands an investigation, which she doesn't expect from Trump's attorney general. "This is an example of all of the president's henchmen, and I hope that the senators do not become part of the president's henchmen."

Senators said later that when Roberts appeared the solemnity of the occasion took hold. Security was tight at the Capitol.

"I thought this is a historic moment, and you could have heard a pin drop," said Republican John Cornyn of Texas. "And so I think the gravity of what are undertaking I think was sinking in for all of us."

Republican House Majority Leader Mitch McConnell took a far different view of the charges and proceedings.

He opened the chamber decrying Pelosi's decision to hand out "souvenir pens" on Wednesday after she signed the resolution to transmit the charges to the Senate.

"This final display neatly distilled the House's entire partisan process into one perfect visual," McConnell said. "It was a transparently partisan process from beginning to end.".

The president suggested recently that he would be open to a quick vote to simply dismiss the charges, but sufficient Republican support is lacking for that.

Instead, the president's team expects a trial lasting no more than two weeks, according to senior administration officials.

It would take a super-majority of senators, 67 of the 100, to convict the president. Republicans control the chamber, 53-47, but it takes just 51 votes during the trial to approve rules, call witnesses or dismiss the charges.

Australian Associated Press