The knives are out for South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE, whose sudden rise ahead of the Iowa caucuses has provoked progressive anger and fresh attacks from Democratic rivals questioning whether the 37-year-old is the right candidate to take on President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE.

Liberals are tearing at Buttigieg, viewing him as a threat to the left's most ambitious policy goals such as "Medicare For All" and free college tuition. Buttigieg also has the full attention of former Vice President Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE’s campaign as the two battle for support from centrists.

While Buttigieg's campaign once basked in glowing media profiles, he is getting tougher scrutiny from the press. Progressives have begun questioning his past consulting work. And a furious debate is raging over whether he will be able to attract enough support from black voters to win the primary.

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Democrats say the attacks against Buttigieg are a sign of his strength in the race and reveal fears from rival campaigns that his rise to the top of the polls in Iowa and New Hampshire could set him on a trajectory to the nomination.

“Pete clearly has a lot of momentum in the presidential race, and that always leads to increased scrutiny from voters and other campaigns,” said Rep. Don Beyer (D-Va.), one of three House members to have endorsed Buttigieg for president.

But Beyer said that Buttigieg "also has the humility and strength to meet that challenge, and to turn it to his benefit by reaching out to new people and growing his support.”

Biden has largely held his fire against Buttigieg, but earlier this week the former vice president accused the Midwest mayor of stealing his health care plan, which builds upon the Affordable Care Act.

Buttigieg responded by saying their plans are "not exactly the same" and that "of course I believe that our approach on health care is the best one."

Former Democratic National Committee official Deshundra Jefferson described Buttigieg's campaign as "a threat" to Biden, saying Biden's camp "should be rightfully worried.”

“Here’s this mayor from small-town Indiana with about 100,0000 people, and he’s really starting to take up oxygen,” Jefferson told The Hill.

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Meanwhile, progressives are raising questions about Buttigieg’s past consulting work at McKinsey & Company.

While Buttigieg released his tax returns from his time at the firm, he has not revealed much detail about his work there, citing a non-disclosure agreement. Buttigieg’s campaign told The Huffington Post this week that they have requested McKinsey release Buttigieg from his clients and from the NDA.

McKinsey has been under fire over a recent report about its work with U.S. Immigration and Customs Enforcement (ICE), and Buttigieg has sought to distance himself from the firm.

“It’s infuriating to see the choices that they have made, especially in years since I left the firm,” Buttigieg said in October.

Buttigieg has risen into the top tier of candidates in recent weeks, but questions remain about his ability to win over the black voters who will be integral to clinching the nomination.

The South Bend mayor has so far struggled to connect with African American voters, who have supported Biden in large numbers.

Buttigieg’s critics have sought to highlight several racially charged controversies he has dealt with in South Bend, including his demotion of the city’s first black police chief. A trial over recorded conversations between South Bend police officers that resulted in the demotion could take place in the summer of 2020.

Buttigieg will face other legal benchmarks next year as well, including a civil rights lawsuit filed by the family of Eric Logan, an unarmed black man who was killed by a police officer in South Bend earlier this year. That investigation could wrap up as early as February, around the time that the first caucus and primary ballots will be cast.

Liberal groups, such as the Progressive Change Campaign Committee, are demanding reporters do more digging into Buttigieg’s record as mayor, calling it a “ticking time bomb” that will go off during the general election and potentially keep black voters at home.

But Buttigieg’s campaign says he’s been candid in addressing his record and blind spots as the white leader of a racially diverse working class town.

The campaign only recently began running advertisements in South Carolina, where black people account for about two-thirds of the Democratic primary electorate, and polls show most voters there don’t know enough about Buttigieg to have an opinion of him.

A new Iowa State University-Civiqs poll that found Buttigieg leading in Iowa also showed up atop the field among non-white voters.

On Thursday, Buttigieg's campaign rolled out endorsements from three former Obama administration officials, including the former president’s "body man," Reggie Love, who compared him to the nation’s first black president.

"A lot of what is said about Pete echoes what critics said about presidential candidate Barack Obama Barack Hussein ObamaBiden says voters should choose who nominates Supreme Court justice Senate GOP set to vote on Trump's Supreme Court pick before election In a season of bad ideas, one stands out MORE — too young, too different, maybe another time — but I believe there is never a better time to fight for change than right now,” Love said on CNN.

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On Wednesday, several local black officials held an event in South Bend where they talked about why they support Buttigieg’s candidacy.

The event was disrupted by a white man who supports Sen. Bernie Sanders Bernie SandersTrump, Biden will not shake hands at first debate due to COVID-19 Sanders tells Maher 'there will be a number of plans' to remove Trump if he loses Sirota reacts to report of harassment, doxing by Harris supporters MORE (I-Vt.). The man took the microphone from a black woman and denounced the proceedings.

The Buttigieg campaign is seething over the encounter, and has used it to rally new supporters online.

“This is what Black folks who support Pete experience all the time on Twitter,” Nina Smith, a black woman who is Buttigieg’s national press secretary, tweeted. “White men grabbing the mic, insulting us, calling us 'uppity,' 'purchased' and 'tokens,' all in support of certain white male candidates. Careful, your racism is showing.”

Supporters have also taken note of the changing tone of coverage around Buttigieg in recent weeks.

The South Bend mayor coasted on momentum from positive coverage early in the primary, receiving fawning reviews as a gay politician and Rhodes Scholar who has served abroad in the military.

But this week, the gay magazine Out knocked him for raising money for the Salvation Army, describing the group as “homophobic.” Vice described Buttigieg’s vision for America as a “caste system.” And a writer for the black website The Root called Buttigieg a “lying motherf-----” over his past comments on minorities and education.

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Buttigieg personally phoned the writer for The Root, saying he “just wanted to listen.”

But the campaign is growing frustrated with some of the attacks, which it views as outlandish and desperate efforts to derail his bid for the nomination.

“If our opponents want to spend the entire campaign melting down about Pete and pushing bad faith attacks against him, be our guest,” communications director Lis Smith tweeted. “He is pretty damn special and the more people learn about him, the more they will like him.”

Other supporters have said the attacks on Buttigieg from the progressive wing of the party are not truly representative of the party’s liberal voters.

“There is a very, very slim vocal minority in the progressive community who are constantly attacking him, but Pete’s campaign is focused on a large swath of progressive that don't necessarily make their arguments on Twitter,” a Democratic strategist supporting Buttigieg’s campaign said.

-- Scott Wong contributed.