WASHINGTON — The U.S. Senate plunged into President Donald Trump’s impeachment trial on Tuesday with Republicans abruptly abandoning plans to cram opening arguments into two late-night sessions and Democrats arguing for more witnesses to expose Trump’s “trifecta” of offenses.

The turn of events was a setback for Republican Senate leader Mitch McConnell and the president’s legal team, exposing a crack in the GOP ranks and the growing political unease over the historic impeachment proceedings unfolding amid a watchful public in an election year.

Chief Justice John Roberts gaveled open the session, with House prosecutors on one side, Trump’s team on the other, in the well of the Senate, as senators sat silently at their desks, under oath to do “impartial justice.” No cellphones or other electronics were allowed.

Opening day stretched into the night. Senators returned to the chamber after 8 p.m. following a dinner break. Republicans turned back Democratic amendments to subpoena documents from the White House and State Department.

McConnell stunned senators and delayed the start of proceedings with his decision to back off some of his proposed rules. Republicans were said to be concerned over the political optics of “dark of night” sessions.

Instead, 24 hours of opening arguments for each side will be spread over three days, swelling Democrats’ momentum as they push to break the standoff over calling new witnesses.

As the visitors’ gallery filled with guests, actress-and-activist Alyssa Milano among them, and Trump’s most ardent House allies lining the back rows, the day quickly took on the cadence of a trial proceeding over whether the president’s actions toward Ukraine warranted removal from office.

“It’s time to start with this trial,” said White House Counsel Pat Cipollone, the president’s lead lawyer, voicing impatience as the proceedings opened in public after weeks of delay.

Cipollone scoffed that the House charges against Trump were “ridiculous,” insisting the president “has done absolutely nothing wrong.”

The White House legal team did not dispute Trump’s actions, when he called Ukraine and asked for a “favor,” which was to investigate Democrat Joe Biden as the U.S. was withholding military aid the ally desperately needed as it faced off with hostile Russia on its border.. But the lawyers insisted the president did nothing wrong. “Absolutely no case,” Cipollone said.

Rep. Adam Schiff, chairman of the House Intelligence Committee, opened for the prosecution saying America’s Founders added the remedy of impeachment in the Constitution with “precisely this type of conduct in mind — conduct that abuses the power of office for a personal benefit, that undermines our national security, and that invites foreign interference in the democratic process of an election.”

Said Schiff: “It is the trifecta of constitutional misconduct justifying impeachment.”

The other lead lawyer on Trump’s team, Jay Sekulow, retorted, “I’ll give you a trifecta,” outlining complaints over the House Democrats’ impeachment inquiry process.

The impeachment trial, unfolding in an election year, is testing whether Trump’s actions toward Ukraine warrant removal at the same time that voters are forming their own verdict on his White House.

All four senators who are presidential candidates are off the campaign trail, seated as jurors. “My focus is going to be on impeachment,” Sen. Bernie Sanders, the Vermont independent, told reporters.

The day began as a debate over rules, and it was only when the clerk started reading the dry language of the resolution that the hand-written changes became apparent.

McConnell made the adjustment after encountering resistance from Republicans during a closed-door lunch meeting. Senators worried about the public reaction to cramming the 24 hours of opening arguments from each side into just two days.

Sen. Susan Collins of Maine and Sen. Lisa Murkowksi of Alaska, along with a substantial number of other Republicans, wanted to make the changes, according to people familiar with the situation. Some senators argued that the two-day limit would have helped Democrats cast Republicans as squeezing testimony through in the dead of night.