The Republicans' 2008 nominee, Arizona Sen. John McCain, teamed up with Romney in South Carolina and invested huge importance in the verdict there.
"If Mitt Romney wins here, he will be the next president of the United States," McCain told the crowd at a century-old peanut warehouse near Myrtle Beach, where the two campaigned with South Carolina Gov. Nikki Haley.
And McCain hammered at Romney rivals Newt Gingrich and Rick Santorum for backing government spending on legislators' "earmark" special projects when they were in Congress, telling voters, "My friends, earmarks are the gateway to corruption."
Romney's GOP rivals are working overtime to cast as him to too timid and too moderate: They're urging Republicans to do themselves a favor and nominate a more conservative standard-bearer offering a sharper contrast to Obama.
"The only way Republicans lose is if we screw this up and nominate another moderate who has taken multiple positions on every major issue of our time," Santorum, the former Pennsylvania senator, told supporters in a fundraising appeal Friday.
Gingrich argued on morning TV news shows that Romney can't win the nomination and said that even if he did, his performance against Obama in the general election campaign debates would be laughable.
The former House speaker, speaking on ABC's "Good Morning America," predicted that Romney would win New Hampshire but that one of the former Massachusetts governor's GOP rivals "will eventually emerge as the conservative alternative and will beat Romney."
Romney is heavily favored to win Tuesday's New Hampshire primary, so much so that he can afford to focus on South Carolina, where voters aren't due to cast primary ballots for another two weeks.
McCain told voters there that Romney could effectively clinch the GOP nomination with a South Carolina win on Jan. 21.
"It's going to come down, as it always does, to South Carolina," he said.
Romney kept his focus on Obama, telling his audience in Conway that the president's proposal to reduce the military and focus more on Asia was "inexcusable, unthinkable and it must be reversed."
His characterization of Obama as a "job killer" didn't get much message reinforcement from the government: The Labor Department reported Friday that employers added a net 200,000 jobs last month and that the unemployment rate fell to 8.5 percent, the fourth straight monthly drop.
Santorum, who has harshly criticized Obama on the economy, said Friday in New Hampshire he was "very gratified" that hiring had picked up. But he added, "There's a lot of concern still."
Santorum, who pulled within a handful of votes to place just behind Romney in Iowa's caucuses, is likely to find a welcome audience among South Carolina conservatives, and so he remained in New Hampshire to try to maintain the momentum he earned from Iowa.
"Don't settle for less than America needs," Santorum asked those expected to vote in New Hampshire's first-in-the nation primary. Without saying so Thursday, he and the other candidates appeared to share a common objective -- hold down Romney's vote totals in New Hampshire, then knock him off stride in the first Southern primary of the year.
Romney benefited handsomely from having several rivals split the vote in Iowa, where his winner's share was roughly 25 percent.
His allies were fully engaged in the tussle over which GOP candidate is the true conservative. On Friday, he showcased the endorsement of conservative leader Bay Buchanan, whose brother Pat won the New Hampshire primary in 1996. Bay Buchanan cast Romney as a "real conservative" who could get things done.
Restore Our Future, the pro-Romney super PAC that unloaded a barrage of negative TV ads on Gingrich in Iowa, planned to go after him again -- this time in print. áThe group announced Friday it had purchased full-page newspaper ads in New Hampshire and South Carolina tying the former House speaker to Obama.
"On issue after issue, Newt Gingrich and Barack Obama have so much in common, the right choice is to choose neither," the ad said, ticking through issues including backing the federal bank bailout and favoring "amnesty" for illegal immigrants
Jon Huntsman, who bypassed Iowa to bet his campaign on a good finish in New Hampshire, was showing off an endorsement by The Boston Globe, Romney's hometown paper. It was the second time Massachusetts' largest newspaper had snubbed Romney ahead of the New Hampshire primary.
Huntsman started his day Friday in Concord, telling a group of about 75 college and high school students that they're "getting screwed" by a country wallowing in debt and suffering what he calls a "trust deficit."
Also vying to emerge as Romney's chief rival were Texans Ron Paul and Rick Perry.
Perry, who finished fifth in Iowa, released a biographical ad in South Carolina that spokesman Ray Sullivan said shows his "perfect-for-South-Carolina status" as a conservative man of faith and a veteran.
Paul, who placed third in Iowa, was arriving in New Hampshire on Friday, in time to campaign and participate in a pair of weekend debates.