Senate Majority Leader Mitch McConnell, R-Ky., arrives at the Senate for the start of the impeachment trial of President Donald Trump on charges of abuse of power and obstruction of Congress, at the Capitol in Washington, Tuesday, Jan. 21, 2020. President Donald Trump’s impeachment trial quickly burst into a partisan fight Tuesday as proceedings began unfolding at the Capitol. Democrats objected strongly to rules proposed by the Republican leader for compressed arguments and a speedy trial. Credit: J. Scott Applewhite | AP

WASHINGTON — Senate Majority Leader Mitch McConnell abruptly changed his proposed rules for President Donald Trump’s impeachment trial, backing off the condensed two-day schedule to add a third for opening arguments after protests from senators, including Republicans.

The trial quickly burst into a partisan fight at the Capitol as the president’s lawyers opened arguments Tuesday in support of McConnell’s plan. Democrats objected loudly to McConnell’s initially proposed rules, and some Republicans made their concerns known in private.

Without comment, the Republican leader quietly submitted an amended proposal for the record, after meeting behind closed doors with senators as the trial opened. He added the extra day and allowed House evidence to be included in the record.

Senators were stunned by McConnell’s shift, and aides offered no immediate answers. But a spokeswoman for Sen. Susan Collins, R-Maine, said she and others had “raised concerns” and the senator sees the changes as “a significant improvement.”

Democrats had warned McConnell’s rules package from could force midnight sessions that would keep most Americans in the dark, with Rep. Adam Schiff, D-California, the chairman of the House Intelligence Committee leading the prosecution, calling the rules a “cover-up.”

McConnell opened the chamber promising a “fair, even handed” process – and warned that the Senate would stay in session until his proposed rules package was adopted. The first test was coming as senators prepared to begin debate and vote on McConnell’s proposed rules.

“The president’s lawyers will finally receive a level playing field,” the Kentucky Republican said, contrasting it with the House impeachment inquiry.

With Trump’s presidency on the line, and the nation deeply divided just weeks before the first Democratic primary contests, four senators who are also presidential candidates will be off the campaign trail, seated as jurors.

The Democrats say the prospect of middle-of-the-night proceedings, without allowing new witnesses or even the voluminous House records of the trial, would leave the public without crucial information about Trump’s political pressure campaign on Ukraine and the White House’s obstruction of the House impeachment probe.

Sen. Mitt Romney, R-Utah, whose votes are being closely watched, said he was satisfied with the proposal, even as he hopes to hear from former Trump national security adviser John Bolton who had a front-row seat to the president’s actions.

House Democrats impeached the Republican president last month on two charges: abuse of power by withholding U.S. military aid to Ukraine as he pressed the country to investigate Democratic rival Joe Biden, and obstruction of Congress by refusing to cooperate with their investigation.

On the night before the trial, Trump’s lawyers argued for swift rejection of the “flimsy” charges in a trial that never should have happened. Trump’s legal team doesn’t dispute Trump’s actions – that he called the Ukraine president and asked for a “favor” during a July 25 phone call.

Instead the lawyers for the president, led by White House Counsel Pat Cipollone and a TV-famous legal team including Alan Dershowitz, say the two charges against the president don’t amount to impeachable offenses and Trump committed no crime. Legal scholars have insisted the framers of the Constitution provided impeachment as a remedy for “other high crimes and misdemeanors,” a broad definition that doesn’t mean specific criminal acts.

Associated Press writers Laurie Kellman, Mary Clare Jalonick, Alan Fram, Andrew Taylor, Padmananda Rama and David Pitt contributed to this report.