According to the 7.20.2009 PP Poll in a 2012 match-up…

Obama 48%, Huckabee 42%
Huckabee -6%

Obama 50%, Gingrich 42%
Gingrich -8%

Obama 51%, Palin 43%
Palin -8%

Obama 49%, Romney40%
Romney -9%

Spread vs. Obama:

From 7/20/2009 Release: http://www.publicpolicypolling.com/pdf/PPP_Release_National_720.pdf