Democrat-leaning Public Policy Polling on Santorum surging into the lead nationally (full results here):

Riding a wave of momentum from his trio of victories on Tuesday Rick Santorum has opened up a wide lead in PPP's newest national poll. He's at 38% to 23% for Mitt Romney, 17% for Newt Gingrich, and 13% for Ron Paul.

Part of the reason for Santorum's surge is his own high level of popularity. 64% of voters see him favorably to only 22% with a negative one. But the other, and maybe more important, reason is that Republicans are significantly souring on both Romney and Gingrich.

...The best thing Romney might have going for him right now is Gingrich's continued presence in the race. If Gingrich dropped out 58% of his supporters say they would move to Santorum, while 22% would go to Romney and 17% to Paul. Santorum gets to 50% in the Newt free field to 28% for Romney and 15% for Paul.