MANCHESTER, N.H. — A squeaker of an Iowa victory behind him, Mitt Romney jumped into the New Hampshire primary, declaring that his staying power sets him apart from caucus runners-up Rick Santorum and Ron Paul and the rest of the GOP presidential field. Michele Bachmann underscored his point by quitting the race, while Rick Perry promised to soldier on to South Carolina.

The withdrawal of Bachmann, who called herself the true conservative of the bunch, left her supporters up for grabs as the pace of the campaign quickens, and that could help Santorum. His appeal to Christian conservatives in recent weeks lifted him from the dregs of the opinion polls to become Romney's top challenger and the conservative of the moment.

"Game on," Santorum declared. But paltry funding and a lean campaign organization will make it tougher for the former Pennsylvania senator to strengthen his momentum.

As a former Massachusetts governor, New Hampshire is Romney's political backyard and he's heavily favored in its Jan. 10 primary. The contests in South Carolina and Florida that pack the final weeks of the month are more promising for the candidates such as Santorum vying for the most conservative voters.

Romney was named Iowa's winner in the wee hours Wednesday — besting Santorum by just eight votes — bringing down the curtain on an improbable first act in the campaign to pick a candidate to challenge President Barack Obama in the fall.

Despite Romney's wealthier, well-oiled campaign, he and Santorum each collected about a fourth of the vote. The Iowa GOP said Romney got 30,015 votes, to 30,007 for Santorum, who racked up support while touring Iowa's rural areas in his pickup truck.

Fourth-place finisher Newt Gingrich said the results show "3 out of 4 Republicans repudiated Mitt Romney. How can you take seriously somebody after that kind of campaign?"

Romney shrugged off the prospect of sharper criticism from his GOP rivals and Obama's re-election team now that he's narrowly affirmed his front-runner status. "I've got a big target on me now," he said Wednesday, adding that it doesn't faze him.

Romney, who finished second in Iowa in 2008 despite a costly effort, campaigned across the state in the closing days of the race, running as a conservative businessman with the skills to fix the economy.

Santorum, Gingrich, Perry and Bachmann argued persistently that Romney wasn't nearly conservative enough on the economy and social issues such as abortion and health care and had vied for months to emerge as the alternative.

Bachmann, the Minnesota congresswoman who finished a distant sixth, announced in Iowa that "I have decided to stand aside."

Bachmann, who was born in Iowa, had won the state's GOP straw poll last summer before plummeting in the polls. Her loss there was devastating. Bachmann said she had run to counter what she called Obama's "socialist policies" and would continue her fight to overturn his health care plan.

Perry, the governor of Texas, told supporters after his fifth-place finish that he was reconsidering his campaign. But Wednesday he announced via Twitter that he was still in the running: "And the next leg of the marathon is the Palmetto State. ... Here we come South Carolina!!!"

Meanwhile, Romney added to his already formidable national network Wednesday with the endorsement of Sen. John McCain in Manchester. McCain twice won the New Hampshire primary — including a defeat of Romney there in 2008 — and was the GOP's most recent presidential nominee.

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."

Jon Huntsman, who skipped Iowa to focus his efforts on a good showing in New Hampshire, dismissed McCain's endorsement with, "Nobody cares."

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, a grinning 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 nonbinding 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 goes forward.

Paul ran third, well ahead of Gingrich, the former House speaker. Both vowed to carry the fight to New Hampshire's primary next week and beyond.

Huntsman 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."

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.

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.

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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.

Paul was trying to demonstrate his third-place finish in Iowa wasn't a fluke. While often dismissed as unelectable by members of his own party, he 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.

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Steve Peoples reported from New Hampshire.