Senate impeachment trial set to begin as Trump adds last-minute reinforcements to defense team

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.