GOP front-runner Mitt Romney is holding a double-digit edge against his rivals nationally, two days ahead of important contests in Southern primaries, according to Gallup's five-day tracking poll.

Gallup's numbers show Romney leading the field with 36 percent support from likely GOP voters, followed by former Sen. Rick Santorum (Pa.) at 24 percent. Former House Speaker Newt Gingrich (Ga.) earns 14 percent in the survey, with Texas Rep. Ron Paul at 11 percent. 

The numbers represent a slight slip from last Sunday, when Romney was at 38 percent, the first GOP candidate to reach that figure since Gallup began tracking the Republican presidential race.

Santorum, who won Saturday's Kansas caucuses, gained 1 point on yesterday's five-day average, with Romney unchanged at 36 percent.

In a head-to-head match-up between him and President Obama, Romney bests the incumbent president with 50 percent support to 46 percent, a two-point bump for Romney and a two-point drop for Obama, since the last survey.

Obama tops Santorum in a hypothetical November contest 49 percent to 48 percent, but those figures reflect a two-point drop for the president and a five-point gain for the former Pennsylvania senator.

Romney, the presumptive nominee, has struggled to keep momentum during the race, but is poised to deliver a knockout blow in Tuesday's contests in Mississippi, Alabama and Hawaii. 

A Rasmussen poll released Friday showed Romney with an eight-point lead in Mississippi, with 35 percent of likely GOP primary voters. Santorum and Gingrich both followed with 27 percent. Polls show Gingrich, Romney and Rick Santorum all in a dead heat in Alabama.

Romney, though, has admitted he faces a challenge in winning over conservative, blue-collar voters in Southern states, calling Tuesday's primaries "a bit of an away game." 

A convincing victory in Mississippi or Alabama would help Romney push out rivals who have positioned themselves as the conservative alternative to the former Massachusetts governor.

Gingrich, who has bet his campaign on performing well in Southern states, also faces a pivotal test Tuesday.

On Sunday, Gingrich predicted two victories. "I think we'll win both. We are campaigning very aggressively in both states," he said."

Gallup's five-day average was compiled over March 6 to 10.