President Trump's impeachment trial kicks off today in the United States Senate. Historian, lawyer, and author of Impeached: The Trial of President Andrew Johnson and the Fight for Lincoln's Legacy David O. Stewart explains the significance of today's events.

The battle over impeachment witnesses has been heating up as the Senate trial begins. So will we see witnesses take the stand? Former Rep. Bob Barr (R-GA) who served as a House manager in the impeachment of President Bill Clinton weighs in on the Democrats' push for additional testimony and evidence for the Senate trial.

Plus, commentary by economist and business professor at the University of Maryland, Peter Morici.