** FILE ** Republican presidential candidate, former Massachusetts Gov. Mitt Romney signs autographs outside his campaign bus after a campaign rally at Tri-City Christian Academy in Chandler, Ariz., Wednesday, Feb. 22, 2012. (AP Photo/Gerald Herbert)

Photo by: Gerald Herbert

** FILE ** Republican presidential candidate, former Massachusetts Gov. Mitt Romney signs autographs outside his campaign bus after a campaign rally at Tri-City Christian Academy in Chandler, Ariz., Wednesday, Feb. 22, 2012. (AP Photo/Gerald Herbert)