Santorum's near-tie with caucus winner Mitt Romney topped a rise from deep in the polls to contender for the presidential nomination.
Meanwhile, Texas Gov. Rick Perry is staying in the presidential race despite a fifth-place finish in the Iowa caucuses.
He's written on his official Twitter account that the "the next leg of the marathon is the Palmetto State. ... Here we come South Carolina!!!"
He's attached a photo of himself jogging near a lake, wearing a Texas A&M running shorts and showing a thumbs-up.
The South Carolina primary is Jan. 21. New Hampshire's is next Tuesday.
Meanwhile, a smiling Romney jetted off to New Hampshire, telling reporters that landslides are terrific but he'd settle for his eight-vote margin of victory.
Romney shrugged off the promise of sharper criticism from his GOP rivals and President Barack Obama's re-election team now that he's narrowly carried the first contest of the nomination.
"I've got a big target on me now," Romney said Wednesday, adding that doesn't faze him. "I've got broad shoulders. I'm willing to handle it."
Fourth-place finisher Newt Gingrich got the attacks off to a quick start, saying the Iowa caucus results show "three out of four Republicans repudiated Mitt Romney. How can you take seriously somebody after that kind of campaign?"
The former Massachusetts governor was declared the winner in the wee hours Wednesday -- by just eight votes -- bringing down the curtain on an improbable first act in the campaign to pick a candidate to challenge Obama in the fall.
Romney and Santorum each collected almost a fourth of the vote. The Iowa GOP said Romney got 30,015 votes, to 30,007 for Santorum, whose late surge carried him to a near win after months languishing in the depths of opinion polls.
"Game on," declared Santorum, jaw set. He easily outdistanced most other contenders to emerge as Romney's top challenger and the conservative of the moment.
Romney added to his already-formidable national network by announcing the endorsement of Sen. John McCain, who twice won the New Hampshire primary and was the GOP presidential nominee in 2008.
In a sign of the acrimony ahead, Santorum said McCain's nod was to be expected and took a jab: "John is a more moderate member of the Republican team, and I think he fits in with Mitt's view of the world."
Romney portrayed himself as the best foil to Obama and said he had the national campaign team and ample fundraising needed to endure the march to the GOP convention this summer. "That's something I think other folks in this race are going to find a little more difficult to do," he predicted. Romney did interviews on all three network TV morning shows Wednesday.
On his campaign plane bound for New Hampshire, Romney told reporters he'd spoken to all his rivals except Gingrich Tuesday night and had gotten only two hours of sleep.
In all, more than 122,000 straw ballots were cast, a record for Iowa Republicans, and the outcome was a fitting conclusion to a race as erratic as any since Iowa gained the lead-off position in presidential campaigns four decades ago.
Returns from all 1,774 precincts showed both Romney with 24.55 percent support and Santorum with 24.54 percent. Texas congressman Paul drew 21.5 percent of the votes.
The results are non-binding when it comes to picking delegates to the GOP convention in Tampa, Fla. But an Associated Press analysis showed Romney would win 13 delegates and Santorum 12, if there were no changes in their support as the campaign wears on.
Paul ran third, ahead of Gingrich, the former House speaker. Both vowed to carry the fight to New Hampshire's primary next week and beyond.
Jon Huntsman, who skipped Iowa but is making a run at New Hampshire, said the "kind of jumbled-up outcome" of the caucuses leaves it an open race.
"Who would have guessed that Rick Santorum tooling around in his pickup truck would have gone from nowhere to practically winning the Iowa caucus?" the former Utah governor said on CBS.
Romney's slim victory also drew Democrats' disdain. Democratic National Committee chairwoman Debbie Wasserman Schultz described him as "limping into New Hampshire."
Romney is heavily favored in New Hampshire's Jan. 10 primary, with contests in South Carolina and Florida packed into the final weeks of the month.
Poised to become the front-runner's chief agitator, Gingrich is welcoming Romney to New Hampshire with a full-page ad in the state's largest newspaper that jabs him as a "Timid Massachusetts Moderate."
The day before, Gingrich, who has repeatedly vowed to stay positive in his party's nomination contest, called Romney a liar on national television. Speaking to supporters later, he made clear that he wouldn't back down.
Paul was joining Santorum and Romney in New Hampshire this week to try to demonstrate his third-place finish in Iowa wasn't a fluke. And the candidates will meet Huntsman, who began ratcheting up Romney criticism of his own in recent days.
Speaking to New Hampshire supporters while the votes were still being counted in Iowa, Huntsman questioned Romney's belief system, suggesting he's "been on three sides of every issue."
Romney has largely ignored the direct attacks so far. He's amassed a ton of money and built a campaign organization in several states that staffers say will be able to go the distance to the nomination. In a show of force Tuesday, Romney became the first candidate to purchase television advertising in Florida, whose primary is Jan. 31.
Some of his competitors -- most notably Santorum -- have given virtually no thought to contests beyond South Carolina's Jan. 21 primary. Santorum struggled to pay for campaign transportation in recent days, never mind television advertising in states beyond New Hampshire.
He's spending just $16,000 to air a television ad on New Hampshire cable stations this week. Romney is spending $264,000 on television advertising in New Hampshire, $260,000 in South Carolina and $609,000 in Florida, according to figures obtained by The Associated Press.
Gingrich doesn't have any television ads reserved going forward. But with two debates set for New Hampshire this weekend, he's likely to use his national audience to drive his anti-Romney message.
And Paul, while often dismissed as unelectable by members of his own party, has strong organizations in states beyond Iowa and is spending more than Romney on television advertising in New Hampshire this week. He's spending roughly $368,000 there and another $127,000 in South Carolina.
Paul told supporters his was one of two campaigns with the resources to do the distance and "believe me this momentum is going to continue."
Despite its importance as the lead-off state, Iowa has a decidedly uneven record of predicting national winners. It sent Obama on his way in 2008, but McCain finished a distant fourth here.
Romney, who finished second in Iowa in 2008 despite a costly effort, barnstormed across the state in the race's final days, running as a conservative businessman with the skills to fix the economy.
Santorum, Gingrich, Perry and Bachmann argued that Romney wasn't nearly conservative enough on the economy and social issues such as abortion. They vied for months to emerge as the alternative.
That effort continues this week, half a continent away.
"We are off to New Hampshire because the message I shared with you tonight is not an Iowa message or an Iowa and South Carolina message," Santorum told his Iowa supporters. "It's a message that will resonate across this land."
Steve Peoples reported from New Hampshire.
More Nation & World Headlines
Yvonne Craig, who played Batgirl in the ’60s, dies at 78 Ex-Subway pitchman to plead guilty to sex acts with minors, child porn Testosterone therapy seems to cut stroke risk Military faces new pressure on gender barriers Navy seeks better sub-hunting technology WWII fighter ace Fritz Payne dies at 104 Hillary Clinton breaks with Obama to oppose Arctic drilling FDA approves ‘pink Viagra’ drug for women
Our new comment system is not supported in IE 7. Please upgrade your browser here.