Santorum, a social conservative, also came within 2 percentage points of Romney in a national Gallup poll, and was ahead by 15 percentage points in Michigan, burberry outlet online where Romney was born and has been expected to do well.
"It's a real threat to the kind of sense of inevitability that Romney was trying to project a few short weeks ago," said Christopher Arterton, a professor at George Washington University.
The Republican candidates are engaged in a state-by-state battle for their party's nomination to challenge Democratic President Barack Obama in the November 6 presidential election.
Conservative voters who are wary of the more moderate Romney may be coalescing around Santorum, who appears to be benefiting from fights between Republicans and Democrats over abortion, gay marriage and contraception in recent weeks.
Almost twice as many Tea Party supporters and white evangelical Christians in the Pew poll chose Santorum over Romney, the former Massachusetts governor.
That survey had Santorum with 30 percent support from Republican and Republican-leaning voters, compared to 28 percent who preferred Romney.
The Gallup poll put Romney at 32 percent and Santorum at 30 percent. Former House of Representatives speaker Newt Gingrich, who led the field as recently as late January, was in third place with 16 percent.
The Public Policy Polling poll result in Michigan is probably most worrisome for Romney, whose father was governor of the state in the 1960s.
Santorum led Romney by 38 percent to 23 percent Burberry Outlet in the Midwestern state, which holds its primary in two weeks.
THE REAL BATTLEGROUND
Arizona also holds its primary on February 28, and Romney is favored to win, but Michigan's primary on the same day is the real battleground for the two leading candidates who are campaigning there this week.
Santorum swept Republican nominating contests in Missouri, Minnesota, and Colorado last week - his first victories since being announced the winner of the Iowa caucuses in January - and has since closed the wide lead Romney held over him.
Campaigning in Arizona on Monday, Romney made no mention of Santorum, instead blasting Obama's budget proposal and touting his own conservative credentials. Romney's staff, meanwhile, has hammered away at Santorum, painting the former senator as a big spender of taxpayer dollars during his time in Washington.
"This election is more than just about replacing President Obama," Romney said at an Arizona campaign appearance. "People are going to say, 'OK, we're disappointed with him, but where are you? Where are conservatives gonna lead?'"
Data for Monday's Gallup poll was based on telephone interviews from February 8 through Sunday of 1,162 registered Republican and Republican-leaning Burberry Bags voters. The margin of error was plus or minus 4 percentage points.