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.
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.