I strongly believe she won't win. Obama is also a consummate politician. He must have calculated that throwing Democratic leadership weight behind Kamala without a semblance of a vetting process would reduce the legitimacy of her nomination, and open her up to attacks based on that. Also, as a Democrat heavyweight who is universally despised by Republicans, he would want to be careful to make his endorsement not look like just one retired negro endorsing another unqualified negro.