Allen, unlike McCain, understands who controls the GOP nominating process -- ie, conservatives.
Allen is smart enough to not let anyone get to his right.
Actually, if another person who is popular with the more conservative primary voters also throws their hats into the ring, it gets that much easier for McCain
If it's just Allen and McCain, McCain would get crushed, but if you have say Frist, Allen, McCain, maybe Huckabee or Sanford, then you are looking at situations in which any given winner, at any given time, could be taking the primary with under 40% of the vote.