President Donald Trump’s impeachment trial quickly burst into a partisan fight as proceedings began unfolding at the Capitol.
Democrats objected strongly to rules proposed by the Republican leader for compressed arguments and a speedy trial.
Chief Justice John Roberts gavelled open the session, senators having taken an oath last week to do “impartial justice” as jurors.
Democrats warned that the rules package from Mr Trump’s ally, Senate Majority Leader Mitch McConnell, could force midnight sessions that would keep most Americans in the dark and create a sham proceeding.
Mr Trump, who has been attending the World Economic Forum in Davos, Switzerland, said the whole thing was a hoax, and he said he was sure it would “work out fine”.
“This is not a process for a fair trial, this is the process for a rigged trial”, Representative Adam Schiff, the chairman of the House Intelligence Committee leading the prosecution, told reporters.
He called it a “cover-up.”
Mr 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 president’s lawyers will finally receive a level playing field,” the Kentucky Republican said, contrasting it with the House impeachment inquiry.
The first test was coming as senators prepared to begin debate and vote on Mr McConnell’s proposed rules.
The package diverges in some ways from the most recent impeachment trial, of Bill Clinton, by condensing opening arguments into two days for each side.
It pushes votes to consider whether or not to hear from witnesses later in the process.
Republican senators, who hold the majority, were falling in line behind his plan.
“Sure it will be a fair trial when you’ve got 24 hours of arguments on both sides,” Senator Chuck Grassley of Iowa told state reporters on a conference call.
The rare impeachment trial, unfolding in an election year, is testing whether Mr Trump’s actions toward Ukraine warrant removal at the same time that voters are forming their own verdict his White House.
Mr Trump himself denounced the proceedings as “a total hoax”, as he does daily, and said: “I’m sure it’s going to work out fine.”
With Mr 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, will leave the public without crucial information about Mr Trump’s political pressure campaign on Ukraine and the White House’s obstruction of the House impeachment probe.
“The McConnell rules seem to be designed by President Trump for President Trump,” said the Senate Democratic leader, Chuck Schumer.
He vowed to call for a series of votes to amend the rules and demand evidence and documents, but it seemed unlikely Republicans would break from the party to join Democrats.
“This is a historic moment,” Mr Schumer said.
“The eyes of America are watching. Republican senators must rise to the occasion.”
If the senators agree to Mr McConnell’s proposal for speedy trial and acquittal, Mr Schiff said: “It will not prove the president innocent, it will only prove the Senate guilty of working with the president to obstruct the truth from coming out.”’