Having energized the left wing of the Democratic Party with his underdog campaign in 2016, Sen. Bernie Sanders is back with another presidential campaign. But this time, the rules of engagement are completely different. CNN Political Director David Chalian sits down with CNN Politics Reporter Greg Krieg to map out what Bernie has done since 2016 to prepare himself for 2020, and what he'll have to do differently going forward if he wants to win the nomination. via Knit