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WASHINGTON — Chief Justice John Roberts and the nation's senators were sworn in Thursday afternoon for the impeachment trial of President Donald Trump.

After being sworn in himself as presiding judge for the trial a little after 2 p.m. ET, Roberts asked the senators to "solemnly swear" to "do impartial justice according to the Constitution and laws."

The senators said "I do" together, and the clerk called senators up to sign the impeachment oath in groups of four.

Two Republican and two Democratic senators — Lindsey Graham, R-S.C.; Roy Blunt, R-Mo.; Patrick Leahy, D-Vt.; and Dianne Feinstein, D-Calif. — escorted the chief justice to the chamber. Roberts entered the room in his judicial robes and was sworn in by the Senate's president pro tempore, Chuck Grassley, R-Iowa.

"God bless you," Grassley said before leaving the dais.

Roberts, who has adamantly defended the nonpartisan role of the judiciary, is unlikely to relish presiding over the trial as senators weigh whether to remove Trump from office.

Asked by NBC News about the mood among his Republican colleagues, Sen. Mike Braun, R-Ind., said as he left the Capitol: "For me, personally, it was a somber feeling, when you listen to the articles and you know how important this is from the historical context. And I think most of us wonder how we've gotten into this twice in 20 years."

Earlier Thursday, the seven House managers chosen to serve as prosecutors in the trial proceeded from the House to the Senate chamber and read the two articles of impeachment.

The managers began the procession through Statuary Hall and the Capitol Rotunda at noon ET. The lead manager, Intelligence Committee Chairman Adam Schiff, D-Calif., then read the two articles on the Senate floor as the other managers faced the dais.

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"Donald J. Trump has abused the powers of the presidency, in that: Using the powers of his high office, President Trump solicited the interference of a foreign government, Ukraine, in the 2020 United States presidential election," Schiff said, reading the text of the first article into a microphone. "He did so through a scheme or course of conduct that included soliciting the government of Ukraine to publicly announce investigations that would benefit his re-election, harm the election prospects of a political opponent, and influence the 2020 United States presidential election to his advantage."

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Proceeding to the second article, Schiff said that the White House directed current and former administration officials not to cooperate with House committees and that, as a result, nine officials defied subpoenas, including acting White House chief of staff Mick Mulvaney.

"In the history of the Republic, no president has ever ordered the complete defiance of an impeachment inquiry or sought to obstruct and impede so comprehensively the ability of the House of Representatives to investigate 'high crimes and misdemeanors,'" Schiff said. "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."

The developments came a day after the managers hand-delivered the articles of impeachment, adopted by the House in December, to the Senate. Just before they transmitted the articles, House Speaker Nancy Pelosi, D-Calif., signed them in a historic engrossment ceremony.

After the swearing-in ceremony Thursday, Senate Majority Leader Mitch McConnell, R-Ky., laid out the next steps. He 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. The Senate trial will resume Tuesday afternoon.

It remained unclear whether witnesses would be called to testify.

Speaking after the proceedings, Senate Minority Leader Chuck Schumer, D-N.Y., told reporters: "This is not something that can be dismissed. President Donald Trump is accused of coercing a foreign leader into interfering in our elections and then doing everything in his power to cover it up.

"This is very, very serious stuff," he added. "This is not trivial. It's hard to imagine a greater subversion of our democracy. I am actually filled with anger."

The House managers who will prosecute the president are Schiff, the lead manager; Jerry Nadler of New York; Hakeem Jeffries of New York; Jason Crow of Colorado; Zoe Lofgren of California; Val Demings of Florida; and Sylvia Garcia of Texas.

Giuliani associate alleges Trump knew

Lev Parnas, the indicted associate of Rudy Giuliani, a personal lawyer for the president, described his role in the Ukraine pressure campaign Wednesday night, pointing a finger at Trump and other top administration officials.

"President Trump knew exactly what was going on. He was aware of all my movements. I wouldn't do anything without the consent of Rudy Giuliani or the president," Parnas said on MSNBC's "The Rachel Maddow Show."

"I have no intent, I have no reason to speak to any of these officials," said Parnas, who faces campaign finance charges and was arrested while trying to leave the country. "I mean, they have no reason to speak to me," he said, referring to Ukraine's current president and other top Ukrainian officials.

At an event in the Oval Office, Trump maintained that he doesn't know who Parnas is, even though Parnas' attorney tweeted pictures and video of them together.

"I know nothing about him, but I can tell you this: I don't know him. I don't believe I've ever spoken to him," Trump said, again referring to the impeachment case as a "hoax."

Here’s the “I don’t know him at all, don’t know what he’s about, don’t know where he comes from, know nothing about him” guy, w Lev Parnas & Roman Nasirov, former head of Ukrainian Fiscal Service, at Mar-a-Lago 12/16. @POTUS .@realDonaldTrump @Acosta #LevRemembers #LetLevSpeak pic.twitter.com/5B5QY2DJEg — Joseph A. Bondy (@josephabondy) January 16, 2020

White House press secretary Stephanie Grisham suggested that Parnas is not a credible witness.

"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," she said. "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."

Schumer called the Parnas interview "astonishing," saying it showed that "crucial pieces of information related to the charges against the president are still coming out."

CORRECTION (Jan. 16, 2020, 5:45 p.m. ET): A previous version of this article misstated Sen. Chuck Schumer's leadership title. Schumer, D-N.Y., is the Senate minority leader, not its majority leader.