Fresh off the slimmest of victories in the Iowa caucuses, Mitt Romney landed here Wednesday and immediately faced intense attacks from Republican presidential rivals who vowed to challenge him more aggressively.

Newt Gingrich held a news conference in Concord to say that Romney is a liberal and a political chameleon, willing to change positions to suit his needs.

Jon Huntsman Jr. dismissed Romney’s newfound support from Sen. John McCain (Ariz.), saying that “nobody cares” about his backing or any of the other high-level endorsements the former Massachusetts governor has received.

And Rick Santorum, who fell just eight votes short of beating Romney in Iowa, said in an e-mail to supporters that Romney is a “bland, boring career politician who will lose to Barack Obama.”

Romney, considered a heavy favorite to win Tuesday’s primary in New Hampshire, sought to dominate the kickoff news coverage and further solidify his standing with GOP voters. He appeared at a midday town hall meeting with McCain to accept his endorsement.

“New Hampshire is the state that will catapult him to victory in a very short period of time,” said McCain, whose win in this state’s primary four years ago had that effect on him.

The focus on Romney came as one of the Republican field’s most vocal conservatives, Rep. Michele Bachmann (Minn.), announced that she was dropping out after her sixth-place finish in Iowa — further narrowing the group seeking to challenge Romney for the party mantle.

Another onetime challenger, Texas Gov. Rick Perry, who finished fifth in Iowa despite spending more than any other candidate, appeared to have decided to soldier on. After returning home Wednesday night to reassess his candidacy, Perry tweeted his intention to head to South Carolina to campaign ahead of that state’s Jan. 21 primary.

After the all-consuming Iowa caucuses, the candidates began to focus on a broader and more complex playing field, with a competitive primary ahead in Florida, on Jan. 31, as well as South Carolina.

And although attacking Romney’s history of centrist positions may not hurt him much in New Hampshire, which is home to many middle-of-the-road Republicans and where independents can vote in the primary, the attacks could resonate in other states where the electorates are more conservative.

More than any rival besides Rep. Ron Paul (Tex.), who finished third in Iowa and has built a network of enthusiastic backers, Romney has the organization and resources to wage an extended national campaign. He has begun airing TV ads in Florida and South Carolina. McCain, a former fighter pilot and prisoner of war, is scheduled to campaign with Romney on Thursday in South Carolina, a military-heavy state where the senator can help boost the candidate’s stature.

Still, a closer-than-expected contest in New Hampshire could damage Romney’s ability to continue making the case to donors and GOP leaders that he is the inevitable nominee. And he must contend here with challenges from all directions, including Paul’s effort to court independents and the presence of Huntsman, who skipped Iowa and has been focusing solely on presenting himself to New Hampshire voters as an alternative to Romney.

“What a squeaker,” Romney conceded Wednesday in a high-school gym here, eliciting chuckles about his razor-thin victory in Iowa. “But it sure is nice to have a win, I’ll tell you. And the question I have for you is: Can we do better here in New Hampshire? Do we think I can get more than an eight-vote margin here?”

Yet even as Romney hoped to squeeze momentum from his Iowa win, an examination of the results revealed his persistent weaknesses among key segments of his party.

Media polling of Iowa caucus-goers found that Romney performed far worse among the most conservative participants — winning 14 percent of them — than he did among those who described themselves as “somewhat conservative” (32 percent) or moderate (38 percent). There was no such ideological divide in his 2008 Iowa tally.

Romney has a related weakness among fervent tea party members, whom he lost to Santorum by a 2-to-1 margin. Santorum quickly capitalized on his success with staunch conservatives, raising $1 million in the day after his near-victory in Iowa, according to his campaign manager, Mike Biundo.

Rivals on Wednesday seized on the apparent gap between Romney and the conservative base of the party.

Gingrich, a former Georgia congressman who has taken in recent days to describing himself as a “Southern conservative,” delivered the hardest blows to Romney. The night before, a visibly angry Gingrich declined to congratulate Romney on his Iowa win — seething over the slew of attacks from the former governor and a super PAC backing him that swiftly knocked Gingrich off his front-runner perch.

“Part of the key will be a distinction between what I’ve actually done as a conservative leader for the last generation and the degree to which in that same time period, Governor Romney was first an independent, then repudiated Reagan-Bush, then voted for Paul Tsongas, the most liberal candidate in the ’92 campaign, then ran to the left of Teddy Kennedy in 1994,” Gingrich said.

He went on to list Romney’s shifts when he ran for Massachusetts governor in 2002, saying that he allowed for “state-funded abortions” in his health-care plan — an assertion that Romney aides deny, saying the policy was created by court precedent and an independent state agency — and appointed liberal judges “to placate Democrats.”

A fundraising appeal from Huntsman’s campaign said Wednesday that the Iowa results showed “the field is completely unsettled and voters are looking for an alternative to Mitt Romney.”

At an evening event, Santorum appeared to take a shot at Romney, although he didn’t mention him by name. Referring to reports that one candidate was considered the most electable, he said: “Don’t buy the media hype. Don’t buy the line that you have to be a moderate to be able to win the election.”

Democrats, too, who have long considered Romney their most formidable general-election foe, sought to take advantage of his standing within the GOP. Referring to his nearly 25 percent take in the Iowa caucuses, which matched his losing performance there in 2008, Obama adviser David Axelrod dubbed Romney “the 25 percent man.”

Romney’s campaign signaled its intention to answer the criticism of his conservatism.

Former New Hampshire governor John Sununu took the stage here with Romney and repeatedly called him a “true conservative.” He highlighted Romney’s Massachusetts record of cutting spending and taxes, standing “shoulder to shoulder” with antiabortion groups, fighting same-sex marriage and opposing his state’s involvement in a regional effort to limit greenhouse gas emissions.

McCain, in two appearances Wednesday, sought to shore up Romney’s credibility as a leader. The senator called the candidate “an honest, straight-talking person of experience, knowledge and vision.”

Through the fall and winter, Romney survived a series of debates relatively unscathed, with his opponents aiming much of their fire on whichever candidate was experiencing a momentary surge in the polls.

But by Wednesday, his team was readying for the battles ahead.

“Let the attacks come,” Romney said on CBS’s “Early Show.” “I’ve got broad shoulders. I know that when you get in a campaign, there’s a big target on you.”

Polling director Jon Cohen in Washington and staff writers Rosalind S. Helderman and Sandhya Somashekhar in New Hampshire contributed to this report.