Santorum lost his last Senate race by 17.4 percentage points — "the biggest for any incumbent senator in Pennsylvania since at least the Civil War."

"That eyepopping margin is the chief reason that few people took Mr. Santorum seriously last year when he started running for president."

Mr. Santorum says he was caught in “a meltdown year” for Republicans...

It was 2006.

But if the climate was harsh, Mr. Santorum was part of it. Always brash, he had become a more rancorous figure since he last faced the voters in 2000. He was No. 3 in his party’s leadership and responsible for its messaging, which often meant either defending Mr. Bush or going on the attack.

And he took high-visibility roles on divisive issues, including abortion, homosexuality and the right-to-die case of Terri Schiavo....

The voters to whom he is appealing this year — mainly conservatives and evangelical Christians — are the same core voters he appealed to in Pennsylvania. But in 2006, they were a minority in the state’s general election; now they dominate the Republican primaries. And they are drawn to Mr. Santorum’s moral certitude, his fire-and-brimstone passion, his pugilistic posture of never giving up and never giving in.