Vermont Sen. Bernie Sanders, who essentially tied with Buttigieg in last week's Iowa caucuses, blasted the former mayor of South Bend, Indiana, for taking contributions from the very wealthy, suggesting Buttigieg won't stand up to "Wall Street tycoons" or "the corporate elite." Massachusetts Sen. Elizabeth Warren voiced similar criticism, telling ABC's "This Week" that "the coalition of billionaires is not exactly what's going to carry us over the top." Former Vice President Joe Biden told the same program that Buttigieg hasn't been able to "unify the black community."

The volley of criticism Sunday was fresh evidence that Buttigieg, who was virtually unknown in national politics a year ago, has become an early front-runner in the battle for the Democratic presidential nomination. The developments usher in a new phase of the campaign that will test how Buttigieg responds to the pressure, especially as the contest moves to more racially diverse states where he has struggled to gain traction.

Buttigieg hit back at Biden, who on Saturday lamented comparisons between the former mayor and former President Barack Obama.

"Oh, come on, man," Biden told reporters. "This guy's not a Barack Obama."

"Well, he's right, I'm not," Buttigieg responded on CNN's "State of the Union." "And neither is he. Neither is any of us running for president."

He later offered an oblique critique of Sanders' combative call for revolution.

"Let's remember we're facing the most divisive president of our time, which is why we can't risk dividing Americans further," Buttigieg told more than 1,800 people at an event in Nashua, New Hampshire.

Later in Dover, he declared himself the candidate on the rise. "We are the campaign with the strongest momentum in the state of New Hampshire, thanks to you," he told a crowd of several hundred.

While responding to some of the attacks, Buttigieg didn't escalate any feuds on Sunday. That could help him maintain the energy of his optimistic Iowa campaign in which he portrayed himself as above the Washington fray.

"Part of the reason why he's doing well is he's got a pretty sunny and upbeat presentation," said David Axelrod, a former senior adviser to Obama. "Tactically, I think it's smart to handle it the way he's handling it. We still don't know what the impact any of this will have."

But in a sign of potential hurdles ahead for Buttigieg, even voters in an overwhelmingly white state like New Hampshire said they wanted to see evidence that he could build relationships with people of color. Kim Holman of Brookline, New Hampshire, said she was undecided but leaning toward Buttigieg's "energy and passion." Yet his struggle so far especially with black voters weighs on her decision.

"It's definitely a concern. New Hampshire is a super-white state," the 52-year-old personal trainer said. "I'm hoping he resonates more with people of color."

Buttigieg's standing has posed a challenge to Sanders. The two contenders represent opposite ideological wings of the party, yet Sanders is under pressure to show that he can unify Democrats if he is the nominee. With that in mind, the progressive Vermont senator has sought to qualify his criticisms of Buttigieg.

When a Sanders supporter in Plymouth laughed at the mention of Buttigieg, Sanders interjected, "We're not here to denigrate Pete."

But Sanders nonetheless proceeded to blast Buttigieg's ties to large donors. And one of his most prominent surrogates, former Ohio state Sen. Nina Turner, ripped into both Buttigieg and billionaire former Mayor Michael Bloomberg at a separate event later Sunday.

She slammed Buttigieg for fundraising with billionaires in a wine cave featuring a Swarovsky crystal chandelier. And she laid into Bloomberg for skipping the early voting states and running a campaign funded by hundreds of millions of dollars of his personal fortune.

"Whose side are you on?" she repeatedly asked the crowd to cheers.

There were other awkward moments Sunday during the final stretch of the New Hampshire campaign. During a rally in the state capital of Concord, Warren declared, "It's up to you, Massachusetts."

During an event in Hampton, a woman asked Biden to explain his underperformance in Iowa. He said it was a good question, then asked her if she'd been to a caucus. When she said she had, Biden responded, "No, you haven't" and proceeded to call her "a lying, dog-faced pony soldier." The audience laughed during the exchange.

The chaos from the Iowa caucuses lingered over the New Hampshire contest. Problems with an app led to delays in results and prompted questions about the accuracy of the vote count. Nearly a week after the caucuses, The Associated Press hasn't declared a result.

In an interview on CNN's "State of the Union" on Sunday, Democratic National Committee Chairman Tom Perez said he was "mad as hell" about the situation.