If Biden pulls off this comeback, Obama and his advisers believe, he will be the one who vouches for Biden’s progressive credentials to the left—despite how much core Sanders supporters have criticized his presidency. If Sanders is the nominee, they think, Obama will be the one who can attempt to legitimize him for the center and other suspicious Democrats.
A pitch for Sanders would not, however, include embracing democratic socialism or pretending that they’re close friends. Obama and his advisers envision a pitch along the lines of “You might not agree with all his positions, but he respects the rule of law,” according to a person who’s spoken with him. He’d contrast Sanders’s values with Trump’s and talk about how, as president, Sanders would run “a government that people can believe in and be proud of.”
Stumping for Joe Biden wouldn’t take as much finesse. The person who’s spoken with Obama envisioned a similar argument about the existential need to get rid of Trump and reinstate norms, but centered on character. Obama would “elaborate on why [he] chose [Biden] to be his VP in the first place, and how over their eight years in office, he became even more confident that he’d make an outstanding president.”