CLEVELAND (Reuters) - A new poll by Quinnipiac University shows Rick Santorum has pulled to the front of the presidential pack with Ohio Republicans who say they are likely to vote in the state's upcoming primary.

Santorum, a former U.S. senator from Pennsylvania, enjoys the support of 36 percent of Republicans who say they will cast ballots on March 6, according to Quinnipiac, followed by Mitt Romney, the former Massachusetts governor, with 29 percent.

Newt Gingrich and Ron Paul pull up the Republican rear with 20 percent and 9 percent, respectively.

Peter Brown, assistant director of the Quinnipiac University Polling Institute, said the picture for Romney has darkened in another way, with more voters in Ohio viewing him unfavorably than favorably.

Still, 50 percent of likely Republican primary voters told Quinnipiac they may yet change their minds before they head to polls on so-called "Super Tuesday."

The survey also found that in Ohio's Senate race, Democratic incumbent Sherrod Brown leads Republican challenger Josh Mandel, 48 percent to 35 percent.

Forty-seven percent of voters said they approve of the job Brown is doing and 46 percent said he deserves a second term in the Senate.

Voters in the state were evenly split over President Barack Obama, with 47 percent saying he should be re-elected and 48 percent saying he does not deserve a second term.

In a general election match-up with Romney, Obama enjoys a slight 45 percent to 41 percent edge with independent voters, the survey found.

Quinnipiac University surveyed 1,421 registered voters, including 553 likely Republican voters, by phone February 7 through 12.

The margin of error was plus or minus 2.6 percentage points, Quinnipiac said.

(Reporting by Kim Palmer; Editing by James B. Kelleher and Paul Thomasch)