Sanders promised to show up at the debate on Sunday in Arizona to face Biden one-on-one, and basically offered him a cheat sheet: "Let me be very frank as to the questions that I will be asking Joe."

Sanders listed off issues central to his campaign, which he says are very important to younger progressives: health care costs, climate change, income inequality, student debt, racial disparities in criminal justice, immigration and housing affordability.

So why tip Biden off to how you're going to challenge him on Sunday? Maybe Bernie Sanders wants Joe Biden to succeed.