Obama, whose victory in Iowa on Thursday transformed him into a political giant slayer, faces five days of heightened scrutiny before the state's Jan. 8 primary amid the klieg lights of sudden superstardom.
Clinton, who just weeks ago was the undisputed Democratic front-runner, must use those same five days to rebound from a crippling loss in Iowa in order to prevent her candidacy from imploding.
"New Hampshire is the last chance for someone who loses Iowa," said Andrew Smith, polling director for the University of New Hampshire.
"You lose in Iowa and you lose New Hampshire, it's done. You go home."
Analysts of all stripes expect a sharp change in tone in New Hampshire after the candidates spent a year playing "Iowa Nice."
Obama's message of hope and unity was also a fine fit for Iowa, a state where voters are notoriously resistant to negative campaigning.
The Democratic contest there was a relatively civil affair, where not a single televised attack ad was aired and where the contenders exchanged only mild jabs.
Such pleasantries will surely be jettisoned in New Hampshire by Clinton and John Edwards, who placed slightly ahead of Clinton in Iowa and who has shown his willingness to take a scalpel to his opponents when necessary.
Edwards staked much of his candidacy on Iowa but aides say he has the resources in New Hampshire to fight on.
All of that means Obama goes into this state's compressed contest with a target on his back - a situation he has managed to avoid throughout his career in politics.
"Obama, through an unprecedented convergence of luck and skill, has never before faced serious attack delivered by a competent opponent," Democratic strategist Dan Newman said. "He's now earned the right to be mercilessly scrubbed and scrutinized. No one knows how he'll respond to the challenge, and how voters will evaluate the criticism."
Clinton, whose once sturdy lead in New Hampshire had already begun to close in the days before Iowa's caucuses, is relying on the state as her husband did in 1992 to make her the "comeback kid."
The former president is still widely popular here and will campaign for his wife until next Tuesday's primary.
Hillary Clinton's aides say her campaign will renew its scrutiny of Obama's comparatively thin record and lack of foreign policy experience, questioning whether he is ready to lead in a dangerous world.
They will also try to paint him as something of a phony - someone whose lofty rhetoric isn't born out in his own public record.
They point to his votes in the Senate to fund the Iraq war even as he tried to position himself as the strongest anti-war candidate in the field.
"He talks about change but has no real record of making change," said Mark Penn, the Clinton campaign's pollster and senior strategist.
The Clinton campaign is also likely to begin airing commercials attacking Obama's health care plan, which they say would leave 15 million people uninsured.
Edwards, meanwhile, can be expected to renew questions about whether Obama's brand of unity politics is too naive for the dog-eat-dog world of partisan Washington.
Much will also be riding on a nationally televised debate among the Democratic contenders Saturday.
Both Clinton and Edwards have typically excelled in such forums, while Obama's performances have been inconsistent.
"The debate will loom large," said Dante Scala, a political scientist at the University of New Hampshire. "It's a chance for Clinton to score some needed points, so she will probably try to make Obama look bad and capitalize on something with regard to his lack of experience."