"I think it is appropriate at this point for me to admonish both the House managers and the president's counsel in equal terms to remember that they are addressing the world's greatest deliberative body," the usually reserved Roberts said. He told them that description of the Senate stemmed from a 1905 trial when a senator objected to the word "pettifogging," because members should "avoid speaking in a manner and using language that is not conducive to civil discourse."

Over and over, Republicans turned back Democratic amendments to subpoena documents from the White House, State Department, Defense Department and budget office. By the same 53-47 party-line, they turned away witnesses with front-row seats to Trump's actions including Mulvaney and Bolton.

Only on one amendment, to allow more time to file motions, did a single Republican, senator Susan Collins of Maine, join Democrats. But it, too, was rejected, 52-48.

"It's not our job to make it easy for you," Adam Schiff, the chairman of the House Intelligence Committee leading the prosecution, told the Senate. "Our job is to make it hard to deprive the American people of a fair trial."

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

Cipollone led the prosecution, scoffing 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 "favour," which was to investigate Democrat Joe Biden as the US 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.

Schiff, the California Democrat, said 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."

"It is the trifecta of constitutional misconduct justifying impeachment," said Schiff.

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 is testing whether Trump's actions toward Ukraine warrant removal as voters are forming their own verdict on his White House.

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

Senate majority leader Mitch McConnell stunned senators and delayed the start of proceedings with his decision to back off some of his proposed rules. He made the adjustment after encountering resistance from Republicans during a closed-door lunch meeting. Senators worried about the political optics of "dark of night" sessions that could come from cramming the 24 hours of opening arguments from each side into just two days.

Collins and Lisa Murkowksi of Alaska, who often buck party leadership, along with a substantial number of other Republicans, wanted to make the changes. It was only when the clerk started reading the dry language of the resolution that the hand-written changes to extend debate to three days became apparent. It also allowed the House impeachment record to be included in the Senate.

The turnaround was a swift lesson as White House wishes run into the reality of the Senate. The White House wanted a session kept to a shorter period to both expedite the trial and shift more of the proceedings into late night, according to a person familiar with the matter but unauthorised to discuss it in public.

Trump's legal team, absent its TV-showcase attorneys, Alan Dershowitz and Ken Starr who were not in the chamber, argued that in seeking new evidence the House was bringing a half-baked case.

But Zoe Lofgren of California, one of the House managers and the first woman to argue for the prosecution in a presidential impeachment trial, said the House wasn't asking the Senate to do the job for them. "The House is asking the Senate to do its job, to have a trial," she said. "Have you ever heard of a trial without evidence?"

The ambassadors and national security officials who did appear before the House delivered often striking testimony, highlights that were displayed on television screens during the Senate proceeding.

Here's Chris Baynes's report.