Rick Santorum has taken the lead nationally in the Republican presidential race for the first time, a new poll showed.

Less than a week after besting Romney in primary contests in three states, Rick Santorum has a 15-point lead on the former Massachusetts governor, according to a national poll released Saturday by Democratic firm Public Policy Polling. Recent polls had shown Romney up about 10 points on Newt Gingrich, his nearest rival.

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Santorum leads Mitt Romney 38 percent to 23 percent in the new poll, while Gingrich is in third place with 17 percent. Ron Paul comes in last with 13 percent.

This is the first major national poll to show Santorum in the lead. The closest he had come previously was after his surprise win in Iowa the first week of January, but even then, Romney performed 10 points better than Santorum nationally.

But Santorum has been riding a wave of momentum since his surprise performance on Tuesday, when he defeated the front-runner not only in Minnesota and Missouri, but also in Colorado, a supposed Romney stronghold. Santorum's campaign has said he has raised more than $2 million since Tuesday, and he was the biggest attraction on Saturday when about 10,000 activists packed the Conservative Political Action Conference in Washington.

Some of Santorum's success in the new poll may be attributed to declining support for both Romney and Gingrich. Romney's favorability rating has declined substantially in PPP's polling and now stands at 44 percent — just one percentage point higher than the 43 percent who say they disapprove. Santorum remains highly popular, with 64 percent saying they approve and just 22 percent viewing him negatively. Gingrich's numbers are almost identical to those of Romney.

Santorum is also besting Romney and the others with key demographic groups, including self-described very conservative voters, Tea Party voters and evangelicals.

"It's important to keep in mind, though, that fewer than half of his voters are firmly committed to him," said Dean Debnam, the polling firm's president. "When he comes under attack in the coming days, his lead could evaporate just as quickly as it was created."

The survey of 656 Republican primary voters was conducted Feb. 9-10 using automated telephone interviews and had a margin of error of plus or minus 3.8 percentage points.