Clinton, a New York senator and former first lady, is seeking to become the first US woman president. Romney, a wealthy former governor of Massachusetts, would be the first Mormon president.
Clinton has reason to be concerned.
Obama, days after winning Iowa soundly over John Edwards, a former Democratic senator, and third-place finisher Clinton, has pulled into a virtual dead heat with Clinton in New Hampshire, according to a Reuters/C-SPAN/Zogby poll released on Sunday.
Talk about change
Clinton inveiled a new strategy a day after a debate in which she sounded frustrated that while Obama talks about changing the US, she believes she has actually carried out change.
"There's a big difference between talking and action, between talking and performing and I am going to make that case to as many people in New Hampshire as I possibly can," she told supporters in Manchester.
New Hampshire is vital to efforts by Clinton and Romney to revitalise their campaigns after disappointing showings in Iowa.
Romney and McCain, winner of the 2000 New Hampshire primary over Bush, are also essentially deadlocked as the White House races in both parties tighten.
Obama, the Illinois senator seeking to become the first black US president, took some swipes at Clinton at Manchester's Palace Theatre, saying he was running for president "not because I feel it's somehow my turn".
"For many months I have been teased, almost derided for talking about hope," he said.
"We saw it in the debate last night when one of my opponents said we can't just offer the American people 'false hopes' of what we can give them. False hopes?"
Romney needs to win or finish high in New Hampshire to maintain his credibility, and is threatened by McCain, the 71-year-old Arizona senator who clashed with Romney at the Saturday debate.
|Giuliani is among those Republicans hoping|
for a victory in New Hampshire [Reuters]
"He talks about changing Washington, Romney said of McCain on "Fox News Sunday".
"But he's been there so long, he's got so many lobbyists at each elbow, he's worked so long - in many cases, he's a maverick against his own party."
McCain, who is competing with Obama for New Hampshire's large section of independent voters, rejected Romney's claim that he has not been for change.
"I'd like to say that I have never been elected Miss Congeniality in the United States Senate because I have tried to eliminate waste and unnecessary spending, defence procurement reform, et cetera," he told CBS' "Face the Nation".
Looking for an edge was Mike Huckabee, a former Arkansas governor, who won Iowa with big support from Christian evangelicals in that state but who may not be able to repeat that in New Hampshire.
Huckabee and McCain both took on Romney at the Saturday debate.
|"The political question in the US is what does that momentum mean going forward? I am sure it does not mean that much"|
Jack Burkman, Republican strategist
Huckabee told "Fox News Sunday" he and McCain have "created a brotherhood here" because both have come under withering attack from Romney.
He said he would be delighted with a third place finish in New Hampshire to propel him into presumably friendlier territory in South Carolina on January 19.
Jack Burkman, a Republican strategist, says the results from Iowa gave little indication of how the presidential race will unfold in the coming months.
"The political question in the US is what does that momentum mean going forward? I am sure it does not mean that much," he told Al Jazeera.
"The message for international audiences is that, this time round, the US primaries have changed in that you have virtually everything at stake on February 5.
"In the past, you had a much more staggered process so that Iowa and New Hampshire meant a lot more. What's going on in the US is the major TV networks are commercialising this for their own ends and profits."
|Huckabee and Obama believe they can win in New Hampshire [AFP]|