After going 0-for-3 in Tuesday's presidential primaries, a defiant Rick Santorum dismissed calls to drop out and predicted he'll win the next contest in his home state of Pennsylvania on April 24.

He'll have to — and not because it would put the former Pennsylvania senator on a path to defeat front-runner Mitt Romney, who has been racking up delegates and is increasingly seen as the inevitable nominee.

A loss in Pennsylvania, where recent polls show Santorum is weakening, would "destroy the rationale for him continuing," says Pennsylvania pollster G. Terry Madonna.

"If he refuses to get out and then loses his home state? That is going to be tough to live down," says Keegan Gibson of the nonpartisan PoliticsPA online news site.

Much can happen in the three-week lag between Santorum's losses Tuesday to Romney in Wisconsin, Maryland and the District of Columbia, and his last real opportunity in Pennsylvania. (Notwithstanding his spokeswoman's attempt Tuesday night to suggest that it's the May primary in delegate-rich Texas that the campaign is really banking on.)

Here's a look at what Santorum faces in the Keystone State he represented in Congress for 16 years.

Desperate Days, Desperate Words

Santorum betrayed his growing desperation Sunday when he attacked Madonna, the longtime and respected Pennsylvania pollster, as a "Democratic hack" after his Franklin and Marshall College poll found the former senator's state support flagging.

Two subsequent polls found much the same: Santorum's lead in the state is narrowing, and Romney is gaining.

"People know that I've been doing this a long time and the polls are accurate," Madonna told NPR. "I think what's motivating the personal stuff is that all of a sudden it looked like he was going to lose three states, and the question gets popped on him about possibly losing Pennsylvania, which is a deal breaker."

"He's also under pressure to get out [of the race], and the whole thing is turning around on him," Madonna said. "He lashed out, and that's not atypical for him."

"His performance here in this state has only been slightly about his national performance — not like Mitt Romney in Massachusetts," Gibson says. "It's not like he's had a solid base among Republicans statewide."

Electability, Gibson says, is the issue for Pennsylvania Republicans, just like everywhere else.

Fading Home-State Edge

Polls released in recent days by Madonna, Quinnipiac University and the Mercyhurst Center for Applied Politics all found a tightening race with Santorum leading. Romney is gaining, and a good swath of Republicans are still undecided.

And exit polls from Tuesday's pivotal contest in Wisconsin showed Romney making inroads with voters previously more attracted to the conservative Santorum — evangelicals and those who identify with the Tea Party movement.

"Here's the real problem for Rick," says Gibson, who worked for former Pennsylvania Sen. Arlen Specter, when he was a Republican and also after he switched his party affiliation to Democrat.

"His weaknesses, which Romney has been trying to play up in other states, would play here," Gibson says. "There's Rick's 2004 endorsement of Specter [over the more conservative Pat Toomey], and Romney's argument that Santorum has 'gone Washington.' "

Both of those issues, he says, crippled Santorum in 2006 when he badly lost his Senate re-election bid to Democrat Bob Casey by a stunning 17 percentage points.

Romney's Strategy

What is occupying some of the Pennsylvania chattering class is the question of what Romney will do, particularly given the complicated method state Republicans use to distribute delegates after the primary.

(Pennsylvania is a "loophole" primary; voters pick not only a candidate, but also delegates who will ultimately commit to a candidate. The system makes it more difficult to immediately claim a specific number of delegates, which is Romney's aim.)

Romney has yet to unleash the advertising furies his campaign and friendly superPAC have used to destroy opponents — particularly Santorum — in other states. The question is, will he? Or will polls suggest to Santorum that he cut some deal with Romney?

"There are several strategies for Romney," Madonna says. "Because the delegate selection here is beyond bizarre, it could well be that he lets Santorum stay here and campaign, and he does the four other states that have primaries the same day."

"Or he could just try to beat Santorum and destroy his candidacy," he said.

Gibson says he's watching to see if Santorum's tone toward Romney "gets softer," and whether there's a full commitment by the former Massachusetts governor to campaign in Pennsylvania. (On Wednesday, Romney has one event scheduled in Pennsylvania; Santorum has three.)

A save-face scenario for Santorum? A possibility, but nobody's betting on it.

It's A Mission

If Santorum's poll numbers in Pennsylvania continue to slide, would he get out of the race before the state's primary?

"No. Absolutely not," Madonna says. "This is not just a campaign in which you win or lose. This has become a mission.

"It's evident in everything he's been doing, particularly in the last six weeks," he said. "He believes that we're on a cliff, and will fall off if Obama wins."

Santorum himself is also at a precipice.

"If he loses [Pennsylvania], it will be a big blow to his stature in the future, but really means very little in the nomination fight now," says William Binning, former chair of the political science department and now professor emeritus at Youngstown State University.

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