After an opening salvo of back-and-forth arguments from President Trump's attorneys and Democrats' impeachment managers on Monday, Senate impeachment trial proceedings are set to begin at 1 p.m. ET on Tuesday with the expectation they will stretch well into a wild night on Capitol Hill -- even as key procedural questions, including the timeline for debate and whether additional witnesses will testify, remain undecided and hotly contentious.

In a surprise move Monday night, a detachment of high-profile House Republicans announced that they would formally join the president's legal team, including Reps. Doug Collins, Mike Johnson, Jim Jordan, Debbie Lesko, Mark Meadows, John Ratcliffe, Elise Stefanik and Lee Zeldin. The last-minute show of force underscored the fluid nature of the Senate trial, which is also set to feature full-throated arguments against impeachment from constitutional scholar Alan Dershowitz and Bill Clinton independent counsel Ken Starr.