It really isn’t racism. Lewis is dumping party bossism that would hand the nomination to someone who didn’t receive a majority of the delegates thru the primary process. Superdelegates will switch rather than steal the election from the people’s choice. The Dem party does not want to commit political suicide. Expect more such defections to occur
It is a racism of convenience with plausible deniability prefactored in, but nobody would seriously consider that these black politicians would be making these statements or bucking the Demorat/Clinton machine if the challenger was say, Lieberman or Zell.