Washington (AFP) - US Democratic candidates galloped into a critical phase of the 2020 campaign Friday after a bruising presidential debate which saw Pete Buttigieg brave incoming fire as a credible emerging threat to his rivals' White House ambitions.

The young mayor of South Bend, Indiana is not the frontrunner -- that title currently belongs to former vice president Joe Biden, who put in a solid performance in the year's final debate.

But Buttigieg's steady rise in recent nationwide polls has brought him attention, including attacks by rivals who see him as an obstacle to their success in early voting states Iowa and New Hampshire.

And so the 37-year-old moderate on Thursday found himself in a debate scrap with his elders, one from the same moderate lane and one in the liberal camp.

To date, Buttigieg's national high water mark was 11.8 percent in late November.

Although he has slid slightly, to 8.3 percent, he has consolidated his status as a genuine contender, leading the polls in Iowa, the state that sets the tone in the primary race with its February 3 vote.

He has also made impressive gains in New Hampshire, which votes the following week.

"This is a fight for Iowa and New Hampshire right now," California Governor Gavin Newsom told NBC after the debate. "Buttigieg is out-performing and, as a consequence, he's at the eye of that storm."

The tempest landed squarely on Buttigieg when Senator Elizabeth Warren clocked him for holding high-price fundraisers.

"Billionaires in wine caves should not pick the next president of the United States," she said, in a memorable reference to a recent closed-door Buttigieg fundraiser in an extravagant California cellar.

Tensions have mounted for weeks between the two candidates, and for good reason, said Democratic strategist Kenneth Baer.

Buttigieg is "cutting into some of the base support of Elizabeth Warren," Baer told AFP, pointing in particular to college-educated white voters.

Buttigieg knew he would get roughed up by rivals, and he came prepared.

"I am literally the only person on this stage who is not a millionaire or a billionaire," he said, accusing Warren, 70, of having a net worth "100 times" his own.

"This is the problem with issuing purity tests you cannot yourself pass," he said.

'Respect our experience'

Amy Klobuchar joined the crackling debate with "Mayor Pete," who leads a city of about 100,000, by chipping away at his skimpy political resume.

"I have not denigrated your experience as a local official," the three-term senator from Minnesota said, appearing to do just that. 

Klobuchar, 59, reminded viewers she did not take kindly to Buttigieg criticizing the establishment experience of candidates like her, Biden and Senator Bernie Sanders in the previous debate.

"I just think you should respect our experience when you look at how you evaluate someone who can get things done," said Klobuchar, who then turned the knife by noting how Buttigieg lost his only statewide race, for treasurer, "by 20 points" in 2010.

But Buttigieg held his own, highlighting his military experience in Afghanistan and his ability to win re-election in South Bend after revealing his homosexuality.

"He kept his composure, he didn't get rattled, and he had an answer," said G. Terry Madonna, director of the Center for Politics and Public Affairs at Franklin & Marshall College.

Klobuchar had her strongest debate performance by far, and the smaller stage afforded her a chance to shine as a "lively and energetic" contender, Madonna said.

But while the top candidates have clear-cut themes, Klobuchar's message is less focused, experts said.

"Klobuchar needs to have a message (beyond) 'I'm from the Midwest, I've won in a tough state several times and that's why you should make me the nominee,'" Baer said. 

Her campaign is polling at about three percent nationally but can expect to gain at least moderate traction after the debate. But while they may cause temporary movement, "they don't produce staying power," Madonna said.

A real test comes in the six-week sprint to Iowa.

Klobuchar launched a 27-county bus tour in Iowa Friday, while Warren returns there on Saturday and Buttigieg arrives Sunday.