@georgeob1,
He isn't going to lose to a primary challenger no matter who might run against him, and, unless I'm mistaken, the article refers to a primary challenge not a 3rd Party challenge in the General Election, so the only way such a challenge can hurt him, is if he pisses off the people who support the challenge (to "make a statement," or "send a message.") by rather blatantly suggesting they are racists or fools ...or foolish racists. I can imagine him doing so, but not that the number of angry liberals who, in reaction, stay home on Nov. 2 will amount to a game changer.
On the other hand, I guess it's possible that such a challenge will roil his African-American supporters who might perceive it as being racist in nature, but here again, I just don't see such a reaction resulting in a significant pick up of votes he couldn't already count on.
Seems to me to be just the sort of meaningless political performance theater that Nader and his ilk consider so profoundly meaningful.