President Donald Trump’s impeachment trial has already begun to unfold at the Capitol Tuesday.

House Democrats impeached the Republican last month due to abuse of power for allegedly withholding U.S. military aid to Ukraine as he the country to investigate Democratic rival Joe Biden, and obstruction of Congress by refusing to cooperate with their investigation.

Matters are complicated further as four presidential candidates are serving as jurors in the Senate trial, keeping them off the campaign trail.

After four days of opening arguments, senators will be allowed up to 16 hours for questions to the prosecution and defense, followed by four hours of debate. These procedures will decide whether extra witnesses will be needed.