the constitution and laws so help you god? >> a historical rarity. we are joined by a professor at columbia university, author of many books. great to have you here as part of our coverage. >> thanks for inviting me. >> what does it mean for history to have this happen. >> well, it doesn't happen very often, as you know. so it puts president trump in a very small category of presidents who have been tried before the senate for impeachment. and in a sense, he will always have that attached to his name, his legacy. so it's -- you know, it is a historic moment in our country. these things don't happen very often. >> what does it tell us that the two impeachments that did occur both felt like procedural aberrations if i may? the johnson case being in theory about a very obscure law, but in practice and reality about racism, civil war, the confederacy. the clinton case also to many