Santorum continues to lead in Ohio polling, though Romney is catching up. Those two states are the biggest delegate prizes on Tuesday. Ohio has the added bonus of being a crucial swing state in the general election with a reputation as the mother of all bellwethers. 

Gingrich, campaigning in both states, in recent days has appealed to frustration among voters with the war in Afghanistan, as protests over the accidental burning of the Koran at a U.S. base coincide with attacks by Afghans which so far have claimed the lives of six U.S. troops. 

Gingrich elicited cheers on Thursday in Georgia when he threatened to withdraw from Afghanistan. On Friday, Gingrich uncharacteristically declared "there are limits to American power." 

"It's time to face the facts. The period where the United States went out and tried to change a civilization which is rejecting that change is over," Gingrich said.