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. Four of the senators sitting in judgment on Trump are running for the Democratic Party's nomination to challenge him in the fall.

"Hear ye, hear ye, hear ye!" intoned the Senate's sergeant at arms, calling the proceedings to order just past noon.

Senators filled the chamber, an unusual sight in itself, sitting silently under strict rules that prohibit talking or cellphones, 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 Constitution mandates the chief justice serve as the presiding officer, and Roberts made the short trip across the street from the Supreme Court to the Capitol. He has long insisted judges are not politicians and is expected to serve as a referee  for the proceedings. Senators rose quickly when he appeared in his plain black robe.

"Will all senators now stand, and remain standing, and raise their right hand," Roberts said.

"Do you solemnly swear that in all things appertaining to the trial of the impeachment of Donald John Trump, president of the United States, now pending, you will do impartial justice according to the Constitution and laws, so help you God?"

The senators responded they would, and then they lined up to sign an oath book.

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 he dismissed the trial anew on Thursday at the White House: "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 the 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 source 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.

"What is the president hiding? What is he afraid of?'' asked Senate Democratic leader Chuck Schumer.

"The gravity of these charges is self-evident," he said. "The House of Representatives has accused the president of trying to shake down a foreign leader for personal gain."

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."

Before the swearing-in, House Democrats prosecuting the case stood before the Senate and Rep. Adam Schiff of the Intelligence Committee formally read the articles of impeachment.

Seven lawmakers, led by Schiff and Rep. Jerrold Nadler of the Judiciary Committee, made the solemn walk across the Capitol for a second day.

All eyes were on Schiff as he stood at a lectern in the well of the chamber, a space usually reserved for senators.

"House Resolution 755 Impeaching Donald John Trump, president of the United States, for high crimes and misdemeanors," he began, reading the nine pages.

The other House prosecutors stood in a row to his side.

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."

GOP Sen. James Inhofe was absent, home in Oklahoma  for a family medical issue, but plans to take the oath when he returns as the full trial begins next week, his office said.

The Senate will issue a formal summons to the White House to appear, with the president's legal team expected to respond by Saturday. Opening arguments will begin on Tuesday.

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. That would be far shorter than the trial of President Bill Clinton, in 1999, or the first one, of President Andrew Johnson, in 1868. Both were acquitted.

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.

A group of four Republican senators is working to ensure there will be votes on the possibility of witnesses, though it's not at all certain a majority will prevail for new testimony.

Sens. Susan Collins of Maine, Mitt Romney of Utah, Lisa Murkowski of Alaska and Lamar Alexander of Tennessee are among those involved.

"I tend to believe having additional information would be helpful," Collins said in a statement. "It is likely that I would support a motion to call witnesses."

Romney said he wants to hear from John Bolton, the former national security adviser at the White House, who others have said raised alarms about the alternative foreign policy toward Ukraine being run by Giuliani.

The House managers are a diverse group with legal, law enforcement and military experience, including Hakeem Jeffries of New York, Sylvia Garcia of Texas, Val Demings of Florida, Jason Crow of Colorado and Zoe Lofgren of California.

Two are freshmen — Crow a former Army Ranger who served in Iraq and Afghanistan, Garcia a former judge in Houston. Demings is the former police chief of Orlando, and Jeffries is a lawyer and member of party leadership. Lofgren has the rare credential of having worked on a congressional staff during President Richard Nixon's impeachment — he resigned before the full House voted on the charges — and then being an elected lawmaker during Clinton's.