“The mayor should be releasing who’s on his finance committee, who are the bundlers who are raising big money for him, who he’s given titles to and made promises to,” Warren said, a rare instance of her directly attacking a Democratic opponent other than Mike Bloomberg by name. Buttigieg, she added, should also “open up the doors so that the press can follow the promises he’s making in these big-dollar fundraisers.”

Buttigieg senior adviser Lis Smith fired back on Twitter, calling Warren a “corporate lawyer” and saying she should open “up the doors to the decades of tax returns she’s hiding.” Warren hasn’t released her tax returns from before 2008, when she had corporate clients while she also taught at Harvard Law School. She disclosed the names of those clients earlier this year but has not released her tax returns from that time, arguing that the decade of tax returns she already released is sufficient.

The exchanges mark a new phase of the primary, particularly for Warren’s campaign. Buttigieg, who’s cutting a center-left path through the primary, continues to rise in Iowa polling, presenting a serious challenge to Warren in a state on which both contenders have staked their candidacies. Biden can potentially afford to place lower than first in the state given his strength in South Carolina, but Iowa is close to must-win territory for Warren and Buttigieg.

Until now, Buttigieg’s jabs at Warren on the debate stage, and in paid ads and the news media, have gone largely unanswered as she’s insisted, “I’m not here to attack other Democrats.”

Still, Warren’s team has bristled at the mayor’s swipes. He has needled her with lines like “fighting is not enough and it’s a problem if fighting is all you have" — a reference to the senator’s frequent calls to arms against conservative adversaries.

But Warren seems to have reached her limit with Buttigieg. Speaking at a Democratic National Committee fundraiser Thursday night, she tried to turn Buttigieg's "fight" critique against him, remarking that “it’s easy to give up on an idea. You can even try to make yourself sound smart and sophisticated when you do it.”

Jeff Link, an Iowa-based Democratic consultant, said the dynamic between the two of them is in plain sight.

“They’re fighting for the top slot in Iowa, which is the center of the universe for them, and it’s crunch time,” he said. The sharpening battle is “a recognition by each campaign that the other is a threat, and we haven’t really had that before now.”

The hostility between the two campaigns has been building beneath the surface. Last week, when Warren was asked about releasing her tax returns, she called out candidates “who want to distract from the fact that they have not released the names of their clients and not released the names of their bundlers.” It was a thinly veiled shot at Buttigieg, who has come under fire for not naming his clients during his time at the corporate consulting firm McKinsey.

Buttigieg has said he’s bound by a nondisclosure agreement and has asked McKinsey to be released from it. He responded by saying that it “sounds like somebody’s changing the subject — is she going to release those tax returns or not? I hope she does.”

Buttigieg is leading the Democratic field with 24 percent in Iowa, according to the RealClearPolitics polling average. A mid-November Des Moines Register poll showed Buttigieg rocketing to the front with 25 percent of the vote, a double-digit lead over the rest of the field. Warren has slipped in Iowa over the past month, after hitting her own high, averaging 23 percent through late September into October.

“As he continues to rise in the polls, he’s going to be the target of more attacks,” said Jennifer Holdsworth, who managed Buttigieg’s failed bid for chairman of the Democratic National Committee after the 2016 election. “Mayor Pete has wide ideological appeal, across the Democratic electorate, from progressives to moderates to everything in between, while others have struggled to communicate a message as widely as him.”

On the ground, Iowa state Sen. Joe Bolkcom, who has endorsed Warren, cautioned that “the Warren campaign needs to make some adjustments in talking to Democratic voters who agree with her on health care, who agree with her on the role that money has played in corrupting politics, but who are nervous about the pace and scale of her agenda.”

“To some degree, Buttigieg has offered a more palatable message to them,” Bolkcom said, on health care and other policies, “and he’s been rewarded here for it.”

Buttigieg’s critics believe the mayor hasn’t been “held to the same standard he has demanded from others,” said Adam Jentleson, a Democratic strategist close to the Warren campaign.

“Pete has been sneaky, demanding that others disclose everything they ever ate for breakfast, while hiding his bundlers and the work for McKinsey that makes up 20 percent of his entire career,” he added. “He has been getting a pass but hopefully that ends now.”

Buttigieg is trying to fend off a range of criticisms, from his time at McKinsey to his failure to appeal to African American voters in South Carolina.

The escalating attacks aren’t limited to Buttigieg and Warren. On a Thursday night interview on MSNBC, Bernie Sanders said “Buttigieg is wrong” in his critique of Sanders’ tuition-free higher-education plan.

“I’m very glad that Mr. Buttigieg is worried that I have been too easy on upper-income people, the millionaires and billionaires, that I’m going to allow their kids to go to public colleges and universities — just [like] by the way, they can go to public schools right now,” Sanders said. “The point is, I happen to believe, that when you talk about programs like Social Security, like health care, like higher education, they should be universal.”

Buttigieg argued on Monday in South Carolina that college “is not for everybody.”

“This is not the same thing as K through 12, this is not the same thing as Social Security,” Buttigieg said. “But where I come from, three out of four people don’t have a college degree, and if the message we’re sending to them is that you need a college degree in order to get by in life, in order to prosper, in order to succeed, we’re leaving most Americans out and I think they’re just missing that very important fact.”