Biden, a former vice president, was blunt when asked by reporters to contrast himself with Sanders, who has long identified as a democratic socialist and was elected as an independent senator from Vermont.

"I'm a Democrat," Biden told reporters. "He's not a registered Democrat, to the best of my knowledge. And Bernie has a different view — I mean everything I've suggested to you that I want to do, I've figured out how to pay for it."

Sanders signed a loyalty pledge with the party last year that acknowledges he's a member of the Democratic Party and would serve as one if elected president.

Buttigieg, meanwhile, bemoaned Sanders' demands for adherence to progressive ideals as "a kind of politics that says you've got to go all the way here and nothing else counts."

The tougher talk comes as a greater urgency sets in among moderates in the race to prevent Sanders from notching early wins in Iowa and New Hampshire. That could give him momentum heading into later contests that will decide who wins the Democratic nomination.

The pressure is growing as moderates struggle to unite behind a clear standard-bearer. While Biden remains atop the field in many national polls, his support has slipped some in the early voting states. Buttigieg and Minnesota Sen. Amy Klobuchar, who have framed themselves as Midwestern consensus builders, have had bigger crowds than Biden across Iowa over the past week and believe they have an opportunity to peel off some of his support.

As the moderates struggle to coalesce, they're also attacking each other, increasingly turning the Democratic primary into a multi-pronged battle.

In addition to his critique of Sanders, Buttigieg dismissed what he characterized as Biden's assertion that it's not time to "take a risk on someone new." That's an implicit argument that the 38-year-old former mayor of South Bend, Indiana, doesn't have the experience needed to be president.

"History has shown us that the biggest risk we could take with a very important election coming up is to look to the same Washington playbook and recycle the same arguments and expect that to work against a president like Donald Trump, who is new in kind," Buttigieg said.

Biden began the day focused on contrasting himself mostly against Trump, arguing, "I don't believe we are the dark, angry nation that Donald Trump sees in his tweets in the middle of the night."

But he couldn't ignore his Democratic rivals entirely. He later issued a warning to voters, saying "we can't let this Democratic race slide into a negative treatment of one another."

"We have differences," Biden continued. "We can argue about these differences. We have to be able, when we come out of this, to unite the party."

Under prodding from reporters during a brief stop for ice cream in Pella, Iowa, Biden responded to Buttigieg's criticisms.

"I don't know what Pete's talking about. He's a good guy, and I'm not gonna get into — he must be deciding things are getting a little tight," he said with a smile.

Asked what he saw as his biggest contrast with Buttigieg, however, Biden noted, "I've gotten more than 8,600 votes in my life" — a reference to the fact that Buttigieg won his 2015 mayoral race with just over 8,500 votes.

Buttigieg has hesitated until now to criticize his opponents by name, though he did repeatedly question Massachusetts Sen. Elizabeth Warren last fall on how she planned to finance a policy to provide health insurance to all Americans.

And while his pivot to associating Biden and Sanders specifically with the "arguments from before" was noticeable, Buttigieg's aides signaled that a shift to a more direct contrast between the candidates was long-planned. The final thrust, the aides said, is to turn doubt about his experience on its head by making the better-known candidates seem risky.

For Biden's part, he's long been reluctant to engage with his opponents, hoping to focus largely on drawing a general election contrast with Trump based on histemperament and character. The campaign believes that approach highlights some of Biden's main advantages with voters: that he's seen by many as the strongest Trump challenger and that voters can connect with Biden on a personal, emotional level.

Iowa Democratic strategist Matt Paul, who ran Hillary Clinton's Iowa campaign in 2016, said he was "surprised" it's taken the candidates this long to go after one another by name — and he warned that Buttigieg's comments could backfire.

"I've actually been surprised, with this many people in the field, with a race as undefined and as fluid as it's been, that others didn't try and find that contrast earlier. It's dangerous to roll out contrasts this late because you do have to be careful, and in Iowa — especially if it gets personal — there tends to be blowback," he said.

Indeed, some voters at Biden's events said they didn't like the negativity among Democratic candidates. Patricia Cooke, a 75-year-old retiree from Newton who had been deciding between Biden and Buttigieg but leaning towards Biden, compared Buttigieg's comments to when Warren sparred with Sanders on stage at the last debate, which she said made Warren look "small."

"I don't like that. it just doesn't make them look good. They should focus on Trump," she said.