Harris: not seeking the death penalty for a cop killer when she a DA, and the Mnunchin stuff. Being a DA/AG I'm sure oppo researchers will dig up some cases where she was lenient on some criminal and take it out of context.
Booker: you can be sure there is something shady going on in NJ. One thing that comes to mind is Zuckerberg's $100M donation to Newark schools that seems to have been misused. And crime rates when he was Mayor.
Warren: Indian heritage.
Sherrod Brown: abuse claim from ex wife
Gillibrand: lawyer for big tobacco, flip flop (blue dog to bold progressive), MattressGirl
Franken: some non PC jokes from years ago about women, handicap etc. but in Trump era means little.
Klobuchar: like Harris as former prosecutor some case will be warped by the GOP.
Franken, Gillibrand, & Klobuchar seem to be 'cleanest' potential candidates in term of vices.
Who has since recanted. And actively campaigns for him. I saw her at a fundraiser last year. His Senate opponent in 2012 tried to use it and got blasted by the ex.