If it’s Sanders vs. Bloomberg, this will be the first time since 1872 that one of the Republican or Democratic nominations has come down to two candidates neither of whom has ever won an election before as a member of that party.
That year, newspaper editor Horace Greeley defeated former attorney general and secretary of state Jeremiah Black for the Democratic nomination. Greeley was already the breakaway Liberal Republican nominee and not a Democrat. He then lost to incumbent President Grant in a lopsided landslide and died a month later, helpfully allowing Congress to establish the precedent that electoral votes can not be validly cast for dead persons.
Cautionary note: I don't study campaigns and elections or voter behavior, so my hot takes are about as reliable as the nattering nabobs on the TV. Don't believe anything I say.