Rick Santorum feels like Rocky Balboa

John King talks with Rick Santorum about his surge in the polls, his strong showing in the Iowa caucuses and how he plans to continue the momentum at 6 p.m. ET Wednesday on “John King, USA.”

Rick Santorum has two words for Mitt Romney.

“Game on,” Santorum said in a speech to supporters after the Iowa caucuses.

With a low-budget, grass-roots campaign that chugged along in near invisibility for months, the former U.S. senator from Pennsylvania caught the Republican establishment completely off-guard with a stunning performance Tuesday night in Iowa. (Romney edged out Santorum by eight votes, but both had roughly 25% of the vote.)

Analysis: Narrow loss a big win for Santorum

Asked whether he felt like another Pennsylvanian, the fictional underdog fighter Rocky Balboa, Santorum could not resist predicting a sequel in the upcoming New Hampshire primary.

“There’s going to be a rematch,” Santorum said to CNN about the battle ahead against caucus winner Romney.

Santorum brushed off news that Sen. John McCain would endorse Romney. McCain was the winner of the 2008 New Hampshire primary and the eventual GOP presidential nominee that year.

“I would have expected that. I’m surprised he hadn’t done it sooner,” Santorum said of McCain’s expected endorsement. Santorum worked in a dig that McCain’s moderate views meshed better with those of Romney.

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Santorum may have been the only politician in Iowa who was not surprised by his remarkable performance.

Even his top Iowa surrogate, Secretary of State Matt Schultz, said he didn’t expect Santorum to do so well.

In the hours before the caucuses began, Santorum sounded cautiously optimistic. But he was careful not to predict victory.

“I feel like if we can crack the top three that would be great,” Santorum told CNN.

5 things we learned from the Iowa caucuses

Santorum earned his strong showing with an old-fashioned, retail-politics campaign. In the months leading up to the caucuses, he held roughly 360 town halls across all of Iowa’s 99 counties.

Unlike his more well-financed rivals, the former senator worked off of a shoestring budget. While some candidates had their faces plastered across slick campaign buses, Santorum rode around in a supporter’s pickup known as “The Chuck Truck.”

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Driving home that down-to-earth image, Santorum often wore his trademark sweater vest and cowboy boots.

All along the way, Santorum was patient. He watched other conservative alternatives to Romney rise and fall. As luck would turn out, Santorum’s moment would come at exactly the right moment.

One week before the caucuses, two polls from CNN/Time/ORC and NBC/Marist confirmed Santorum was gaining support.

Then Saturday’s Des Moines Register poll, which has a solid track record of predicting the outcome of the caucuses, confirmed the Santorum surge. Over the four days during which the poll was conducted, Santorum finished third. But voter preferences measured during the last two days found Santorum in second place, statistically tied with Romney.

In a fresh sign of Santorum’s strength, the former Massachusetts governor took a soft jab at his newfound rival.

“Like Speaker Gingrich, Sen. Santorum has spent his career in the government in Washington,” Romney said.

Speaking to an overflow crowd jammed into a Polk City coffee shop, Santorum fired off a surprisingly sharp response, taking aim at Romney’s business experience.

“I know one of my opponents who has now directed his attention to me, surprisingly, has said that he has executive experience. We are not looking for a chief executive officer for this country. We are looking for a commander in chief,” Santorum said.

In the day before the caucuses, Santorum told another crowd in Boone he was the subject of negative robocalls that accused him of opposing the Second Amendment. Santorum blamed another rival, U.S. Rep. Ron Paul of Texas, for being behind the calls.

There were also challenges coming from conservative media outlets, where right-leaning pundits questioned Santorum’s viability in a general election. In 2006, Santorum lost his Senate seat to Democrat Bob Casey by 18 percentage points.

Santorum brushed off the concern. “I lost an election in the worst election year for Republicans in the history of our state,” Santorum said.

A major factor in Santorum’s defeat was his controversial vote to have a judge intervene in the case of Terri Schiavo, a Florida woman who was living in a vegetative state at the time and whose family was deeply divided over whether to remove her feeding tube.

Santorum defended his vote to CNN.

“What I cared about with Terri Schiavo was that a judge looked at the case fairly and they did. And they made their decision,” Santorum said.

“I didn’t think that was an unreasonable thing to do.”

Polls conducted before Santorum’s re-election defeat found voters in Pennsylvania were opposed to congressional intervention in the case.

Santorum’s handling of the Schiavo case did agree with social conservatives, who are now flocking to his campaign.

One day before the Iowa caucuses, the former senator picked up the endorsement of Jim Bob Duggar, a reality TV star and Christian conservative who backed Mike Huckabee in 2008. Huckabee won the Iowa caucuses with the support of evangelical votes.

Duggar and roughly half of his 19 children accompanied Santorum on stops Monday across Iowa.

The reality-show family traveled in a Santorum for President bus.

In the final moments when nearly all of the votes were in, Santorum was 19 ahead of Romney.

People at the Santorum caucus party joked it was the Duggar family that had put him over the top.