When the other Democratic candidates lost support in the 08 primaries, Obama, Edwards and Clinton and later (Obama and Clinton) readily gained most of that support. Those two candidates were in most Democratic voters top 1 to 3 choices. When Bachmann, Perry and now Herman Cain are taking turns supplanting a candidate as a front runner, that candidate has to be concerned about their constituency not being that enamored of them. You are already outside of the top choices for most of the voters for the GOP nomination.