There is a significant number of democrats who will never vote for Hillary. There is an almost equal number who will never vote for Obama. These are the mccain democrats because they would rather vote for mccain if their preferred democratic candidate, either Hillary or Obama is not the nominee.
The Latinos and the Asians in my state will never vote for Obama because they are McCain democrats. The rednecks in Lubbock will never vote for Hillary because they are McCain democrats.