"I've got a big target on me now," Romney said, adding that doesn't faze him. "I've got broad shoulders. I'm willing to handle it."
The former Massachusetts governor was declared the winner of the leadoff presidential caucuses in the wee hours Wednesday by just eight votes, bringing down the curtain on an improbable first act in the campaign to pick a challenger to President Barack Obama in the fall.
Appearing hours after the caucuses had ended, Iowa GOP chairman Matt Strawn said Romney had 30,015 votes, to 30,007 for Santorum, whose late surge carried him to a near win.
Earlier, 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 jabbed at his rival. "John is a more moderate member of the Republican team, and I think he fits in with Mitt's view of the world," he said.
"Game on," declared Santorum, jaw set, after easily outdistancing several other contenders to emerge as Romney's unvarnished conservative rival for the primaries yet ahead.
Even before his victory was announced, Romney looked past his GOP rivals and took aim at Obama. "The gap between his promises four years ago and his performance is as great as anything I've ever seen in my life," he told supporters in Iowa's capital city.
On Wednesday, 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.
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 jumbled 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. 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 and former House Speaker Newt Gingrich was fourth, and both men vowed to carry the fight to New Hampshire's primary next week and beyond.
Not so Texas Gov. Rick Perry, who came in fifth and told supporters he would return home to Texas to reassess his candidacy.
Minnesota Rep. Michele Bachmann was a distant sixth, and her campaign appeared in disarray. She told reporters she would carry on -- less than an hour after her campaign manager raised doubts in an Associated Press interview about whether she would stay in the race.
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.
Gingrich knocked Romney as "a Massachusetts moderate who in fact will be pretty good at managing the decay, but has given no evidence in his years in Massachusetts of any ability to change the culture or change the political structure or change the government."
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 Jon Huntsman, the former Utah governor, who skipped the Iowa caucuses and 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." Earlier in the week, Huntsman criticized Romney's ties to Wall Street donors and congressional Republicans.
Romney has largely ignored the direct attacks so far, saving his own criticism for the man he hopes to face next November, Obama. 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. "There's going to be an election up in New Hampshire, and believe me this momentum is going to continue and this movement is going to continue and we are going to keep scoring," he said.
Despite its importance as the lead-off state, Iowa has a decidedly uneven record when it comes to predicting national winners. It sent Obama on his way in 2008, but McCain finished a distant fourth here to former Arkansas Gov. Mike Huckabee.
Romney, who finished second in Iowa in 2008 despite a costly effort, initially campaigned cautiously this time.
But he barnstormed extensively across the state in the race's final days in pursuit of a first-place finish, 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 to the former Massachusetts governor.
And they'll continue that effort 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.