The second impeachment trial of Donald Trump begins on Tuesday as Senate leaders agreed a rapid timetable.
The former president is charged with “inciting insurrection” in a speech to supporters ahead of the deadly riot at the US Capitol on 6 January.
Democrats say they have “overwhelming” evidence of his guilt while his defence team claim rioters acted independently.
The defence call the trial “absurd and unconstitutional” and its legality of it will be a focus on Tuesday.
Mr Trump is the only US president in history to have been impeached twice and one of only three to have been impeached at all.
If convicted, he could be barred from holding office again. But a two-thirds of the 100-seat senate is required to convict – Democrats hold 50 seats and there is little sign enough Republicans will join them.
In a pre-trial statement released on Monday, Mr Trump’s lawyers argued the trial is unconstitutional because Mr Trump has left office and is now a private citizen.
The nine “impeachment managers” – Democrats from the House of Representatives who will lay out the case for prosecution – dispute this, saying he should be accountable for actions as president.
Both sides will have four hours to tackle this constitutional question on Tuesday, with a vote expected at the end of the day on whether to proceed.
Senate leaders agreed a framework for the trial on Monday.
“All parties have agreed to a structure that will ensure a fair and honest Senate impeachment trial of the former president,” US Senate Majority Leader Chuck Schumer said.
If Tuesday’s constitutional vote passes – and it is expected to – opening arguments will begin on Wednesday afternoon with both sides allowed up to 16 hours each for presentations.
These arguments are expected to run through until the weekend when senators will get a chance to ask both sides questions.
It is unclear if the impeachment managers will then extend the timetable by requesting witnesses be called or subpoenaed – though Mr Trump has already declined to voluntarily testify.
Lawmakers on both sides are said to favour a quick trial, amid an ongoing effort to have President Joe Biden’s coronavirus relief package approved.
With the speedy timetable, it is thought a Senate vote on whether to convict Mr Trump or not could come as early as Monday.