WASHINGTON — Before he began his presidential campaign, Mayor Pete Buttigieg called his political following the “Happy Warrior Movement,” a label intended to convey an upbeat and collaborative striving for ambitious goals.
That call for unity became a signature part of Mr. Buttigieg’s bid for the White House. In the September debate, Mr. Buttigieg, the mayor of South Bend, Ind., interrupted a testy exchange between former Vice President Joseph R. Biden Jr. and the former housing secretary Julián Castro to warn that going on the attack against fellow Democrats would be counterproductive.
“This reminds everybody of what they cannot stand about Washington, scoring points against each other, poking at each other, and telling each other that — my plan, your plan,” Mr. Buttigieg said. “Look, we all have different visions for what is better.”
One week later, though, Mr. Buttigieg shifted his approach. He called Senator Elizabeth Warren of Massachusetts “evasive” on her plans for paying for “Medicare for all,” kicking off a monthslong clash between the Harvard graduate and the former Harvard Law professor.
For Mr. Buttigieg, the strategy of going on the attack has largely worked. He didn’t cement his place in the top tier of the Democratic primary until he became more aggressive.
His aides say they have been successful in drawing Ms. Warren and Senator Bernie Sanders of Vermont into advantageous policy fights on free college and eliminating private health insurance, which Mr. Buttigieg opposes. And in Iowa, where Democrats famously say they don’t like negative campaigning, Mr. Buttigieg has not been punished for going on the attack — in fact, he has been rewarded.
But on Thursday, Mr. Buttigieg may be in for some payback. As the polling leader in Iowa, the first caucus state, he is likely to draw substantial fire from most of the six other Democrats debating onstage at Loyola Marymount University in Los Angeles.
If so, it would be Mr. Buttigieg’s first experience as the primary punching bag, an honor that has gone to Mr. Biden and Ms. Warren in previous debates.
During the November debate, Mr. Buttigieg largely got a pass from his rivals in a sleepy affair in Atlanta. But since then he has been the subject of attacks from Ms. Warren on his fund-raising practices, from Mr. Sanders on health care policy and from other Democrats skeptical of his postcollege work for the McKinsey consulting firm and his ability to appeal to African-American voters.
Since September Mr. Buttigieg has mounted a sustained onslaught — most of it aimed at Ms. Warren, but also against Mr. Sanders and Mr. Biden, with occasional shots at other candidates now far beneath him in the polls.
“His pressures on Warren and some of the farther left policies is expanding the Democratic base, allowing some more moderate people to feel like they have a voice in this race,” said Bryce Smith, the Democratic Party chairman in Dallas County, Iowa.
But they have also dovetailed with a growing resentment from rival candidates as they have seen him surpass them in the polls.
Meanwhile, his record appealing to African-American voters has come under increasing scrutiny, testing his electability in a contest in which beating President Trump is of utmost importance to Democratic voters.
Here’s a look at how Mr. Buttigieg has gone after his leading rivals, and what they have said in response.
Buttigieg vs. Warren
Mr. Buttigieg has leaned into his attacks against Ms. Warren with more enthusiasm than any other rival. In Iowa, the two are fighting for a similar bloc of voters — college-educated whites who are paying close attention to the campaign. As a result, the contrasts he has crafted with her have been more charged and personal than any other conflict in the campaign.
Back in September, Mr. Buttigieg began attacking Ms. Warren as “evasive” because she hadn’t yet released details of her health care plans.
Within a span of three days in October he got under the skin of the Warren campaign with two distinct attacks. On the eve of the CNN/New York Times debate, he belittled Ms. Warren’s policy of not holding closed-door high-dollar fund-raisers by saying Democrats won’t defeat Mr. Trump with “pocket change.” Then after the debate he told CNN that “last night she was more specific and forthcoming about the number of selfies she’s taken than about how this plan is going to be funded.”
By November, after Ms. Warren had released the health care details Mr. Buttigieg had called for, he said “the math is certainly controversial” in one interview and in another called her plan “divisive.”
After refraining for months from by-name counterattacks — without naming Mr. Buttigieg, she had argued in early November that “consultant-driven campaigns” were sure losers — and watching Mr. Buttigieg surpass her in Iowa polls, Ms. Warren had finally had enough. Earlier this month, at a Democratic National Committee fund-raiser, she demanded that Mr. Buttigieg allow reporters into his closed-door fund-raisers.
“Mayor Pete should open up the doors so that anyone can come in and report on what’s being said,” Ms. Warren told reporters in Boston. “No one should be left to wonder what kind of promises are being made to the people that then pony up big bucks to be in the room.”
Four days later the Buttigieg campaign announced he would allow reporters into his fund-raising events and release a list of campaign bundlers.
She has since moved on to attacking Mr. Buttigieg’s “Medicare for all who want it” health care proposal, which would keep in place the existing health insurance industry Ms. Warren and Mr. Sanders have pledged to dismantle.
“His plan is not offering full health care coverage to anyone,” she told reporters in Iowa on Monday. “His plan is still about high deductibles, about fees, about co-pays and about uncovered expenses. What I’m offering is full health care coverage.”
Buttigieg vs. Sanders
When he was 18, Mr. Buttigieg wrote an award-winning essay lauding Mr. Sanders. When he joined the presidential race, Mr. Sanders was among the first rivals he attacked, saying in an April interview that Mr. Sanders couldn’t win a general election.
By September, Mr. Buttigieg was denouncing the “Sanders-Warren vision” of health care in TV interviews, then promoting that contrast in digital ads on his Twitter feed.
His Iowa TV ads in September for the first time mentioned Mr. Sanders and Ms. Warren by name. More recently, he suggested in a CBS News interview that his past praise for Mr. Sanders was merely a product of his youth.
“You know, the Sanders campaign definitely has more young voters,” he told CBS. “I was a big fan of Bernie Sanders when I was 18 years old.”
And last week Mr. Buttigieg repeated a suggestion he first made during the summer that the Medicare for all proposal that Mr. Sanders has championed for years would put millions of insurance industry employees out of work. He made the accusation during an interview with Rachel Maddow in which he defended his time working as a management consultant for McKinsey & Company.
“There are some voices in the Democratic primary right now who are calling for a policy that would eliminate the job of every single American working at every single insurance company in the country,” Mr. Buttigieg said.
Like Ms. Warren, Mr. Sanders had avoided firing back at Mr. Buttigieg until recently, preferring to let online allies like Jacobin magazine do that work for him. But last weekend in Iowa Mr. Sanders fired a shot of his own, saying Mr. Buttigieg wishes to preserve an unfair health care system.
“If you maintain a system where millions of people continue to get their private, their insurance from their employers, the average worker in America making about $60,000 a year is paying $12,000 for their health care,” Mr. Sanders told a crowd in Burlington, Iowa. “That’s 20 percent of somebody’s income. If Buttigieg or anyone else wants to maintain that system, I think that is really unfair to the working families of this country.”
Buttigieg vs. Biden
Mr. Buttigieg has made fewer direct attacks on Mr. Biden than on his more progressive rivals. The two share a base of older voters, and each has sought to chart a more moderate path to the Democratic presidential nomination.
Instead, Mr. Buttigieg, 37, has drawn an implicit contrast with the 77-year-old former vice president. He speaks about forging a new political era and dismisses long-tenured Washington politicians as part of the nation’s problems he aims to solve.
In November, he told donors at a California fund-raiser that black voters were sticking with Mr. Biden because of “familiarity” and not because he is “the candidate with the best answers on the subject of race,” according to a report in The Intercept. Weeks later, speaking to Fox News, he said Mr. Biden’s edge with black voters was not “permanent.”
Mr. Biden, who has a tendency to be condescending to younger people who question his record, has so far refrained from calling Mr. Buttigieg a young whippersnapper.
But on his campaign bus in Iowa this month, Mr. Biden told reporters that Mr. Buttigieg “stole” his health care plans and suggested that reporters had gone soft on the South Bend mayor.
“What would you have done to me?” Mr. Biden asked. “You’d have torn my ears off.”
Sydney Ember contributed reporting from Burlington, Iowa.