Barack Obama is hitting his stride, just as Hillary Clinton's Democratic juggernaut stutters, setting up a punch-for-punch 18-day run-in to the first White House nominating clash.
The race is a tense, statistical dead heat in Iowa, before the fabled nominating caucuses on January 3, and a dogfight in New Hampshire, which votes five days later.
Most surveys show Obama rising, and the longtime frontrunner sliding, suggesting that the young senator's timing may be spot on.
An Obama spurt which began in Iowa, now appears to be spreading, raising doubts about the notion that Clinton can afford a loss in Iowa, due to a lead in national polls and a 'firewall' in later states.
"The impression starting to form out here is the same," said Dante Scala, a professor at the University of New Hampshire.
"From Hillary Clinton having a large lead, to Hillary Clinton basically being in a statistical tie -- a one point lead is not a very high wall."
Should the Illinois senator win the nomination, an encounter this week between the foes on an airport tarmac in Washington may come to be seen as the moment when history's course was set.
Clinton, who once reigned over the Democratic field, had to offer an embarrassing personal apology, after an aide raised Obama's dabbling in drugs as a wayward teenager, in her campaign's latest clumsy attack on him.
Hours later, in the last pre-caucuses Democratic debate, Obama displayed a new confidence, and brushed aside Clinton, who's previous imperiousness was replaced scrappy new persona.
Obama, former Democratic senator Bob Kerrey told The Los Angeles Times, has "either peaked, or he's on a trend line that is going to make him the nominee of the party."
Whether this is an Obama boom, or an Obama bust will likely be decided by the rivals' exhausting treks through Iowa in the next 18 days.
Clinton aims to solidify her support among women and older voters who are most likely to caucus. Obama hopes to confound predictions that his legions of young voters will not show up on caucus night.
The former first lady is putting on a brave face, despite the indignity of having to deny her campaign is disintegrating.
"Politics now is a 24/7 cycle you go up you, go down" she told Iowa public television Friday.
"I have always known this was going to be a challenge for me."
On the upside, Clinton knows Iowans are notoriously late in making up their minds.
But there can be few Americans, let alone Iowans who have not formed an opinion of her by now -- so she may also have to worry her support has peaked.
Most of the campaign's recent misteps, the drugs remarks, a much mocked email about Obama setting his sights on the White House in kindergarten, reflect the difficulty Clinton has had in attacking her rival.
Iowans disdain gutter politics so "going negative" on Obama is a huge risk.
Obama knows that, and Saturday sought the moral high ground.
"There's a history of politics being all about slash and burn and taking folks down and what I recall the Clinton's themselves calling 'the politics of personal destruction,'" he said.
"My suspicion is that that's just not where the country is at right now."
One beneficiary of a Clinton/Obama slugest might be John Edwards, a threat in the state, who hopes to surge between them for an unlikely win.
A retooled Clinton strategy has emerged in recent days: first, sow doubt about Obama among Democrats who fear again nominating a punching bag for Republicans; second, contrast Clinton's years in public life, with Obama's perceived inexperience.
"I have been tested, I have been vetted ... there are no surprises," Clinton said Friday, but denied she was hinting at hidden Obama skeletons.
Ex-president Bill Clinton warns that nominating Obama would be a risk.
"When is the last time we elected a president based on one year of service in the Senate before he started running?" he asked in an television interview Friday.
Clinton strategist Mark Penn warns Democrats could squander a moment of political promise, by nominating a novice.
"If you go back and look at the great change presidents, they have been people that had the experience," he said, citing Franklin Roosevelt and Lyndon Johnson.
"They knew how to make change happen."