Mitt Romney leads the GOP field nationally, but Rick Santorum is once again closing in, according to Gallup’s latest five-day rolling average.

Romney took 35 percent support from likely GOP voters, followed by Santorum at 29 percent, Newt Gingrich at 13 percent, and Rep. Ron Paul (Texas) at 10 percent.

Romney opened up a 16 point lead over Santorum in early March, but Santorum has been gaining ever since.

Santorum’s resurgence was helped in part by two primary victories in the Deep South on March 13. Romney seemed poised to post a victory in either Mississippi or Alabama, and his campaign raised expectations leading up to the contests, however, the former Massachusetts governor finished third in both.

None of the candidates have been able to crack the 40 percent mark in Gallup’s national polling this year. Romney hit 38 percent while opening up his big lead in early March, but has since gravitated back towards Santorum.

It’s the fifth time this primary season that a candidate has registered in the upper 30s, only to fall back off those highs.

The latest state polling data suggests that Romney is headed for a big victory in Illinois on Tuesday, while Santorum leads the most recent polls in Louisiana, where primary voters will cast ballots on Saturday.

(Source: The Hill)