My best advice to any Republican candidate is wait another 4 years. Obama's the wrong man to go up against. It'll be like going up against FDR, Kennedy or Clinton. The Obama campaign, when it gets going, will charm the pants off the voter, and his charisma will be enough to see him through, even if his domestic record is shakey. Few Presidents have much look with domestic, thats more a Senate's toy, a President lives and dies by his foreign policy, and he killed Bin-Laden for ultimate brownie points.
Also, America doesnt really like changing its President too much, Since 1932, Only Bush snr, Carter and Ford have failed to get a second term when seeked (Ford, and to a large extent Carter, only really got a first due to the Watergate fallout)