WASHINGTON – The first substantive day of President Donald Trump’s impeachment trial opened Tuesday with unexpected internal GOP dissension over its structure, as Senate Majority Leader Mitch McConnell, R-Ky., was forced to revise his proposed rules at the last minute to accommodate a brewing rebellion in his ranks.

Under the updated guidelines, both sides will receive more time to present their cases, and findings from the House impeachment probe will automatically be entered into evidence as part of the trial.

The concerns over the rules underscored the narrowness of the majority that McConnell will have to tend to as Trump’s impeachment trial proceeds, while also balancing the demands of a White House that has repeatedly pushed for a swift acquittal of the president.

The Republican dispute unfolded as the House impeachment managers and the president’s legal team took to the floor of the Senate chamber to lay out their cases on the two charges against Trump: abuse of power and obstruction of Congress related to his alleged attempts to pressure Ukraine to dig up dirt on a political rival.

Rep. Adam Schiff, D-Calif., the chairman of the House Intelligence Committee, told senators that the most important question confronting them was whether to accept a structure that Democrats said would not produce a fair trial as he argued for Trump’s removal from office.

“If the House cannot call witnesses or introduce documents and evidence, it’s not a fair trial,” Schiff said. “It’s not really a trial at all.”

In response, the president’s lead lawyers also pressed the issue of fairness, arguing that House Democrats had rushed through a process that deprived Trump of his due process rights and that Trump should not have been impeached at all.

“A partisan impeachment is like stealing an election. And that’s exactly what we have,” White House Counsel Pat Cipollone said. “Talk about the framers’ worst nightmare. It’s a partisan impeachment that they delivered to your doorstep in an election year.”

Two points in McConnell’s initial resolution that established how the trial would be run drew concern from several Republicans: a provision that would have compressed 24 hours of opening arguments into two days for each side and language that would not automatically admit evidence from the House impeachment probe unless there were a formal vote to include it. In a closed-door party lunch before the trial day began Tuesday afternoon, a number of Republican senators protested those portions of the resolution, according to multiple officials familiar with the meeting.

GOP senators who raised questions about the evidence issue were concerned that it would give Democrats easy ammunition to portray Republicans as being unfair, according to two of the officials, who spoke on the condition of anonymity to describe the senators’ sentiments. Many Republicans were also upset that pushing 24 hours of opening arguments into two days for each side – a restriction that was not included in the rules for President Bill Clinton’s trial in 1999 – would give the appearance that the GOP was rushing the trial.

Sen. Mike Braun, R-Ind., said Republicans thought that “if we did try to rush it or compress it, that that might not be the best thing to do.”

One of the senators most upset about the initial provisions was Sen. Susan Collins, R-Maine, who will be a key swing vote on procedural matters during the trial. But the concerns spread beyond a closely watched core of potential GOP swing senators, with Sen. Rob Portman, R-Ohio, also among those voicing objections in the closed-door lunch about the two-day timetable. That provision didn’t please Sen. Lisa Murkowski, R-Alaska, either. One Republican official with direct knowledge of the discussion described it as “lots of dissension and chaos.”

Democrats seized on the changes as proof that a small clutch of GOP senators facing public opposition to any component of the highly contentious trial could force McConnell to reverse course, though the Republican leader was expected to win all 53 GOP votes in favor of his measure by the end of the day.

“The public realizes how unfair the McConnell proposal is, and the pressure that we have put on them and on Republican senators has gotten them to change,” Senate Minority Leader Charles Schumer, D-N.Y., told reporters. “There are still many other things that are unfair, particularly in relationship to witnesses, particularly in relationship to documents.”

Now, both the Democratic impeachment managers and Trump’s defense team will have up to three days each to make their case, and evidence from the House will be entered automatically unless there is an objection. The changes were so last-minute that there were handwritten scribbles in the legislation marking the revisions.

The White House had initially requested condensing the opening arguments into two days for each side, according to several people familiar with the drafting. One senior administration official said it had done so while cautioning that the language could change depending on the needs of Trump and various senators, and that the number of days for opening arguments was a lower priority for the White House.

On other issues, Republicans killed attempts from Democrats to modify the parameters of the proceeding to allow for an assortment of documents and a slate of witnesses at the outset.

In a sign of fatigue, at least on the GOP side, McConnell halted the trial proceedings shortly before 9:30 p.m. in a bid to negotiate an end to the hours-long debate that seemed destined to go on for much longer. But after a brief recess that allowed for senators to talk, the two sides did not reach a deal to speed things up.

“There will be a good number of votes,” Schumer said. “We are willing to do some of those votes tomorrow. There is no reason we have to do them all tonight and inconvenience the Senate and the chief justice. But we will not back off on getting votes on all of these amendments, which we regard as extremely significant and important to the country.”

Earlier in the debate, Schumer and Senate Democrats forced an attempt to subpoena documents kept by the White House – including the National Security Council – as well as a separate effort to obtain records from the State Department and a push to get similar information from the Office of Management and Budget. All were rejected on party-line votes.

Rep. Zoe Lofgren, D-Calif., one of the seven House impeachment managers, argued that the White House was sitting on hundreds of pages of relevant materials. She played video of key House witnesses who said they had submitted notes about discrepancies in the rough transcript of Trump’s call with Ukrainian President Volodymyr Zelensky on July 25 that has been at the heart of the case against the president – as well as a core part of his defense.

“Attorney-client privilege cannot shield information about misconduct,” Lofgren said.

Trump was impeached Dec. 18 by the House on charges that he held up nearly $400 million of congressionally appropriated aid to Ukraine and a coveted White House visit for Zelensky as a condition for the Eastern European nation launching an investigation into former vice president Joe Biden – a potential 2020 rival for the White House – and his son, Hunter.

The president’s attorneys have argued that the requests from Trump were related to his concerns about corruption. Hunter Biden had served on the board of Burisma, Ukraine’s largest private gas company, whose owner came under scrutiny by Ukrainian prosecutors for possible abuse of power and unlawful enrichment, although Biden was not accused of any wrongdoing.

Amid the vigorous debate about his Ukraine dealings, Trump was in Davos, Switzerland, meeting with other global leaders and touting U.S. economic prosperity at the World Economic Forum.

In brief remarks to reporters in Davos, Trump dismissed his impeachment as a “hoax.”

“It goes nowhere because nothing happened. The only thing we’ve done is a great job,” Trump said. “That whole thing is a total hoax, so I’m sure it’s going to work out fine.”

About 45 minutes after the Senate trial began, Trump tweeted in all caps: “READ THE TRANSCRIPTS!” – part of his persistent argument that the record of his call with Zelensky absolves him of wrongdoing.

Through the hours of arguments, all 100 senators sat silent at their desks, barred from their smartphones and with many of them taking prodigious notes. Among the members of the public that were spotted in the half-full galleries in the Senate chamber were former senator Jeff Flake, R-Ariz., and actress and activist and actress Alyssa Milano.

At one point, Cipollone sought to focus attention on four senators who are seeking the Democratic presidential nomination but who must be in attendance at the trial, saying that “some of you are upset because you should be in Iowa right now.” Several senators cast glances toward one of those candidates, Sen. Bernie Sanders, I-Vt., who was watching Cipollone’s remarks with his elbows on the arms of his chair, hands folded in front of his face.

Both Cipollone and Jay Sekulow, one of Trump’s outside attorneys, relayed inaccurate statements during the trial.

Sekulow claimed that the White House was not allowed to have a lawyer present during the impeachment hearings spearheaded by the House Judiciary Committee; the White House was allowed to participate but declined to do so. Cipollone also falsely argued that House Republicans had no access to the secure facilities where closed-door depositions of key House witnesses were taken late last year.

House impeachment managers wrote to Cipollone earlier Tuesday suggesting he may have a conflict that should keep him from representing Trump. In the letter, the House managers noted several instances in which it appeared that Cipollone was aware of Trump’s conduct regarding Ukraine.

The White House swiftly dismissed the concerns, with White House spokesman Hogan Gidley calling Democrats an “utter joke.”

“The idea that the counsel to the president has to turn over protected documents and confidential information is ludicrous, and to imply he can’t represent the president of the United States in an impeachment proceeding is completely absurd,” Gidley said.

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The Washington Post’s Karoun Demirjian, Colby Itkowitz, Paul Kane, Elise Viebeck, John Wagner and Amy B Wang contributed to this report.

Video: Rep. Adam B. Schiff, Democrats’ lead impeachment manager, criticized Senate Majority Mitch McConnell’s proposed rules for President Trump’s impeachment trial on Jan. 21.(The Washington Post)

Video: Republicans and Democrats on Jan. 19 argued about the best way to achieve a “fair” impeachment trial for President Trump in the Senate.(REF:kellym/The Washington Post)

Video: President Trump called the Senate impeachment trial against him a ‘total hoax’ while speaking to reporters at the World Economic Forum in Davos, Switzerland on Jan. 22.(The Washington Post)

Video: In opening remarks at the Senate impeachment trial on Jan. 21, House manager Adam B. Schiff (D-Calif.) said the American people do not believe the Senate will hold a fair trial of President Trump.(The Washington Post)

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Impeachment and President Trump