US:Hillary Rodham Clinton, her status as the front runner for the Democratic presidential nomination in jeopardy, has stepped up attacks on her closest rival with fewer than six weeks until the first nominating contest in Iowa.
On the Republican side, the debate also got sharply negative. With Rudy Giuliani looking to spring a surprise against Mitt Romney, the two candidates traded accusations about taxes, crime, immigration, abortion and ethical standards.
Just weeks ago, Ms Clinton chastised her opponents for "mudslinging". But she unapologetically pursued her main challenger, Barack Obama, over the weekend, standing by her decision to mock his foreign policy experience and attacking his health-care plan, part of what her advisers described as a new phase of her campaign that will present voters with a "real choice".
"I think that there are differences among us on issues and on qualifications and on experience - and voters are going to begin drawing those judgments," Ms Clinton said, in response to a question about whether Democrats should attack each other. She proceeded to hammer Mr Obama over his healthcare proposal, saying that only her approach would ensure coverage for all Americans, and mocking him for what she called a "kind of confusing" approach to healthcare.
Both candidates are locked in a tight race in Iowa with former senator John Edwards of North Carolina, and each is putting renewed focus on electability - a factor that helped turn the state for John Kerry in the 2004 Democratic contest.
Although most Democrats at the national level view Ms Clinton as the most viable nominee, Iowans are more receptive to viewing Mr Obama and Mr Edwards that way. All of the campaigns concede that electability is a top concern among caucus-goers. Health plans and war policy aside, they want to back a winner.
Strategists for Mr Obama said they saw an opening for their candidate on the question of electability, and campaign manager David Plouffe also predicted a "relentlessly negative" barrage from the Clinton campaign in the days ahead.
Central to the new Clinton push will be the argument that only she can beat the eventual Republican nominee, a claim Mr Obama is also seeking to make to voters here. Advisers said her message will be: "You can't have change if you don't win."
Her rivals, meanwhile, are moving aggressively to capitalise on Ms Clinton's weaknesses in Iowa - and, they hope, block her path to the nomination.
In New Hampshire Mr Romney leads in the Republican polls but Mr Giuliani now believes he has a chance to derail the former Massachusetts governor's campaign before it can build the kind of momentum that could make him unstoppable.
Leading in national polls, Mr Giuliani had long appeared to be playing down the importance of early-voting states such as Iowa and New Hampshire in favour of the bigger states that hold their contests in late January and early February. But he said in an interview at the weekend that he intended to win here. "We think we can catch him [ Mr Romney] and get ahead of him," he said.
Mr Romney responded by saying Mr Giuliani sounded increasingly worried about losing the nomination. "He's not in the top three in Iowa and he's not in the first two in New Hampshire, so desperate times for Mayor Giuliani call for desperate efforts."