Hard-charging White House hopefuls have a last chance to cut down their rivals in face-to-face debates this week, just three weeks before voters begin weighing in on 2008 party nominees.
Surging Mike Huckabee will be in the crosshairs when Republicans gather in midwestern Iowa Wednesday, and Hillary Clinton will try to check the momentum of her top foe Barack Obama when Democrats clash at the same venue Thursday.
With polls suggesting cliffhanger struggles in both parties ahead of the Iowa caucuses on January 3, the dual debates may be pivotal.
"The debates will be closely watched by the Iowa electorate and will possibly tip things one way or the other -- in the undecided categories particularly," said Professor Bruce Buchanan of the University of Texas.
Republican Mitt Romney may have the most to lose Wednesday, as probably his only viable route to the nomination rests on victories in Iowa and New Hampshire, which holds primary elections January 8.
But after pouring millions of dollars into the state, he has seen Huckabee, a sunny former Arkansas governor and ordained Baptist minister, roar past him with a come-from-nowhere surge in opinion polls.
Huckabee is bracing for a ferocious grilling, amid unflattering scrutiny of his past life in the bear-pit politics of Arkansas.
Most recently, he has been fighting off a controversy over a 1992 statement that AIDS patients should be quarantined, telling Fox News he would probably "say things a little differently" now.
The Democratic debate will also be carried across the midwestern state's public television network.
Clinton, for months the prohibitive party pace-setter has seen gaping leads in crucial states evaporate, as her younger rival Barack Obama, fresh from a campaign swing with talk show diva Oprah Winfrey, gathers speed.
The debate should allow Clinton to shine, as she outperformed her rival in most campaign debates so far.
But a stumble in a previous clash in Philadelphia, over illegal immigration, catalyzed a slow erosion of the aura of inevitability around her campaign.
Clinton's best hope may lie in a velvet glove rather than a bid to bludgeon Obama, analysts said.
"I think if she can do it in a way that doesn't come across as mean or angry ... (she will try to) show up Obama's inexperience," said Dante Scala, of the University of New Hampshire.
Clinton argues she encapsulates both experience and change, trying to neutralize first-term senator Obama's powerful voice of hope.
While Obama is closing the gap on Clinton in some key polls, he has rarely shined in debates, sometimes coming across as long-winded and professorial in contrast to his fiery rhetoric in formal speeches.
"In some of the early debates it was clearly the case," said Buchanan.
"More recently, it was less so ... Hillary's strength right now is something that people are weighing against Obama's new direction."
Clinton's campaign rejects perceptions her White House bid is increasingly fragile under Obama's pressure -- but a clear win this week in Iowa would help her far more than political spin.
"The conventional wisdom changes from week to week -- for sure a good week could turn things back," said Scala.
The debate will also be crucial for John Edwards, the defeated vice-presidential nominee in 2004, banking on a strong performance in Iowa to stay alive in the race.
Latest polls among Republicans show Huckabee's numbers rising even outside his Iowa stronghold.
A CNN/Opinion Research poll Monday found him chasing down long-time national front-runner Rudolph Giuliani, backed by 22 percent of Republican voters nationwide, compared to 24 percent for the former New York mayor.
A CBS News/New York Times poll had the former Arkansas governor on 21 percent of party primary voters, a point behind Giuliani, a stunning achievement after polling just four percent in October.
Both polls gave Clinton a solid double digit national lead over Obama.
But weekend polls showed races narrowing to just a few points in New Hampshire and South Carolina, another key early voting state.