Santorum, a former U.S. senator from Pennsylvania, opened a 10 percentage-point lead over the former Massachusetts governor, 36 percent to 26 percent, results the Gallup tracking poll released Tuesday indicated.
Former House Speaker Newt Gingrich received 13 percent and U.S. Rep. Ron Paul of Texas had 11 percent, the Princeton, N.J., polling agency said.
In a separate Gallup-USA Today survey, participants chose Romney over Santorum as the candidate with the best chance of beating President Obama in November, 54 percent to 29 percent.
However, nearly three in 10 Republicans in the Gallup-USA Today survey who said they thought Romney was the best candidate against Obama still said they would vote for Santorum over Romney.
Republican tracking results are based on nationwide telephone interviews of 1,194 Republican registered voters as part of Gallup Daily tracking conducted Wednesday through Sunday. The margin of error is 4 percentage points.
Results of the Gallup-USA Today poll are based on nationwide telephone interviews with 1,014 adults conducted Thursday through Sunday. The margin of error is 4 percentage points.