MSNBC's Ari Melber hosts "The Beat" on Wednesday, May 15, and reports on the upcoming Biden-Trump presidential debate, Donald Trump's criminal trial, and backlash to House Speaker Mike Johnson's appearance in Trump's courtroom. Plus, Chai Komanduri and Baron Davis join for the latest "Fallback" installment. Kristy Greenberg, Joe Trippi, and Mark McKinnon also join.