U.S. Chief Justice John Roberts (L) arrives at the Senate for the impeachment trial against President Donald Trump, in Washington D.C., the United States, on Jan. 16, 2020. U.S. Chief Justice John Roberts was sworn in on Thursday afternoon on the Senate floor for the long-awaited impeachment trial of U.S. President Donald Trump, the third president in U.S. history to face such a trial in the upper chamber. (Photo by Ting Shen/Xinhua)

WASHINGTON, Jan. 16  -- U.S. Chief Justice John Roberts was sworn in on Thursday afternoon on the Senate floor for the long-awaited impeachment trial of U.S. President Donald Trump, the third president in U.S. history to face such a trial in the upper chamber.

The trial is set to begin on Tuesday with Roberts presiding.

SENATORS SWORN IN

After taking his oath, Roberts swore in 99 of the 100 senators who will serve as impeachment jurors with the oath to deliver "impartial justice."

Senator James Inhofe, a Republican from the western state of Oklahoma, was absent Thursday reportedly for a family medical issue.

"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?"

Together the senators responded "I do" at their desks and then were called up to sign the impeachment trial oath book in groups of four.

After the swearing-in ceremony, Senate Majority Leader Mitch McConnell gave the House until Saturday and the White House counsel until Monday to deliver trial briefs outlining their arguments, allowing a House deadline for rebuttal the following day.

Trial rules approved by the Senate on Wednesday prohibit senators talking or using cellphones during the trial proceedings.

According to the U.S. Constitution, the chief justice serve as the presiding officer. Senators will ultimately render the verdict.

IMPEACHMENT ARTICLES

Prior to Roberts and senators being sworn in, House impeachment managers, all Democratic lawmakers, came to the Republican-run Senate at noontime on Thursday and formally read the two articles of impeachment accusing Trump with abuse of power and obstruction of Congress.

"With the permission of the Senate, I will now read the articles of impeachment," said lead manager Adam Schiff, chairman of the House Intelligence Committee, on the Senate floor.

"House Resolution 755 Impeaching Donald John Trump, president of the United States, for high crimes and misdemeanors," Schiff began, reading the nine-page impeachment document.

The first impeachment article accuses Trump, by using the powers of his high office, solicited the interference of a foreign government, Ukraine, in the 2020 U.S. presidential election to his advantage.

Proceeding to the second article on obstruction of Congress, Schiff said the White House directed former and current administration officials not to cooperate with House committees. and as a result, nine officials have defied House subpoenas.

"This abuse of office served to cover up the president's own repeated misconduct and to seize and control the power of impeachment - and thus to nullify a vital constitutional safeguard vested solely in the House of Representatives," Schiff said.

FURTHER DEVELOPMENTS

Earlier on Thursday, the Government Accountability Office (GAO), a top government watchdog, said the Trump administration violated the law by withholding congressionally approved military aid to Ukraine last summer "for a policy reason".

The Office of Management and Budget (OMB) withheld some 214 million U.S. dollars in funds appropriated by Congress to the Defense Department for security assistance to Ukraine "for a policy reason, which is not permitted under the Impoundment Control Act," the GAO said in a report.

"The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA," the watchdog concluded.

One day earlier, Lev Parnas, the indicted associate of Trump's personal lawyer Rudy Giuliani, told MSNBC of the role he played in the Ukraine pressure campaign, claiming "President Trump knew exactly what was going on. He was aware of all my movements."

In response, White House Press Secretary Stephanie Grisham said Thursday morning: "These allegations are being made by a man who is currently out on bail for federal crimes and is desperate to reduce his exposure to prison."

"The facts haven't changed - the president did nothing wrong and this impeachment, which was manufactured and carried out by the Democrats has been a sham from the start," said Grisham.

THRESHOLD IN U.S. HISTORY

"We are here today to cross a very important threshold in American history," House Speaker Nancy Pelosi said on Wednesday as the Democrat-run House of Representatives voted to appoint and authorize seven managers for the Trump impeachment trial and to send two impeachment articles to the Senate.

"Here we go again, another Con Job by the Do Nothing Democrats," Trump tweeted earlier on the day.

It remains unclear if any witnesses will be called. Senate Minority Leader Charles Schumer and other Democrats have pressed for witnesses like former National Security Advisor John Bolton to be called to testify during the trial, while McConnell has argued against their demands.

McConnell has previously said that he wants to pass two resolutions, one at the outset of the trial about the rules, and the other passed after opening arguments and questions from senators, which would determine who will be called as witnesses, similar to the trial of former President Bill Clinton.

White House Counsel Pat Cipollone is expected to lead Trump's defense, with the president's personal attorney Jay Sekulow at his side on the Senate floor, said a CNN report.

Trump was alleged to have pressed Ukrainian President Volodymyr Zelensky into launching investigations that could politically benefit him. Furthermore, the White House allegedly tried to cover it up after Pelosi initiated an impeachment inquiry in September.

A whistleblower raised concerns about the White House's interactions with Ukraine in an anonymous complaint last summer. Trump has denied any wrongdoing, repeatedly calling the impeachment "a hoax."

Under the U.S. Constitution, the House shall have the sole power of impeachment, while the Senate shall have the sole power to try all impeachments.

Conviction can only happen in the Senate and requires at least two-thirds of its members, or 67 senators, to vote in favor after a trial. Currently, the Senate has 53 Republicans, 45 Democrats and two independents.