WASHINGTON — The U.S. Senate opened the impeachment trial of President Donald Trump with quiet ceremony Thursday — senators standing at their desks to swear an oath of “impartial justice” as jurors, House prosecutors formally reciting the charges and Chief Justice John Roberts presiding.

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.