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.