Former New York city Mayor Michael Bloomberg appeared on Face the Nation on May 18. He said an independent can never be elected President because there are too many voters who will never consider abandoning their favorite major party’s nominee. See this story.
He might have also mentioned that Article Two of the U.S. Constitution says that if no one gets a majority in the electoral college, then the U.S. House chooses the president, with each state’s delegation getting one vote. The members of the U.S. House would probably not be likely to vote for someone other than their own party’s nominee.