I think Barack prizes intellectual acuity and he probably respects only Liz and Pete on that dimension among the 2020 candidates. Of the two, Liz puts out more policy heft because she has spent more time dealing with public policies on a large scale; Pete simply has not. But Liz also gave Barack some headache with the policies she advocated in the aftermath of the 2008 Great Recession. Pete has not done anything then. Barack could endorse Hillary in 2016 (not that he actually did until the Primary ended), no one could credibly accuse Hillary for being an intellectual/policy light weight. The other wildcard is Bloomberg; he has his baggage from his time as NYC mayor, but no one can credibly accuse Bloomberg for being stupid. I cannot see Barack being too enthusiastic about endorsing any one else, even though he will endorse whomever is chosen at the end of the Primary.
