Try?
Teachout and Nixon are obviously not members of the guys, so they get endorsed against.
Clinton went pretty easy on Bernie because she didn't see her as a threat until late, you could say the same for Bernie, he didn't go hard on Hillary until he thought he could win late in the game.
If Biden had jumped in, he would've been immediately identified as a threat and she would've made it her mission to denigrate him to the harshest extent possible, and she would've stayed until the bitter end even if it meant Bernie winning the nomination as a result of the centrist vote being split.