WASHINGTON (Reuters) - If either Elizabeth Warren or Amy Klobuchar is going to win the 2020 Democratic U.S. presidential nomination, they will have to take down Pete Buttigieg to do it.

That seemed to be the calculation the two senators made at the debate in Los Angeles on Thursday, when they blasted the 37-year-old mayor of South Bend, Indiana, over his wealthy supporters and his thin political resume.

Buttigieg, who remains an unknown to many Democratic voters, had seen little hostility on earlier debate stages. But he has been rising in opinion polls, particularly in Iowa, where he leads the Democratic field with just over six weeks before the first votes of the 2020 campaign.

That threatens the political fortunes of Warren and Klobuchar, who like Buttigieg have premised their campaigns on performing well out of the gate in Iowa.

Buttigieg’s rise there seems to have largely come at Warren’s expense, with both candidates appealing to white, college-educated voters.

As a result, the U.S. senator from Massachusetts increasingly has begun sparring with the mayor. The bad blood between the two on the campaign trail spilled out into the open on Thursday for Democratic voters to see.

Buttigieg held up well under the scrutiny - and may have benefited from the time in the spotlight.

“Anytime you are the topic of discussion, it’s a good thing,” said Kelly Dietrich, a Democratic strategist who trains future candidates for office and helps neophyte candidates break through to the public. “Any press is good press.”

Michael Ceraso, a Democratic strategist who worked for U.S. Senator Bernie Sanders’ 2016 presidential bid and briefly worked for Buttigieg earlier this year, argued Warren and Klobuchar had made a tactical error.

South Bend Mayor Pete Buttigieg asks to respond to a question towards the end of the sixth 2020 U.S. Democratic presidential candidates campaign debate at Loyola Marymount University in Los Angeles, California, U.S., December 19, 2019. REUTERS/Mike Blake

“His opponents are foolish to even give him the time of day,” Ceraso said. “Ignore him; that’s how you beat him.”

Buttigieg is a fundraising powerhouse, which has allowed him to build a campaign organization in Iowa that can compete with Warren’s and that of her fellow progressive, Sanders.

As he has advanced in the polls, Buttigieg has increasingly drawn a contrast with those left-leaning senators, casting himself as more of a moderate in the mold of former Vice President Joe Biden, who was also on stage in Los Angeles.

Buttigieg’s criticism of the single-payer Medicare for All healthcare plan backed by both Warren and Sanders has spurred enmity among their supporters, especially on social media where he is a frequent target.

While Warren spurns big-ticket fundraisers, Buttigieg has embraced them as a means of amassing the resources to win the nomination and take on Republican President Donald Trump in November’s election.

Warren in turn has suggested Buttigieg’s reliance on wealthy donors is symptomatic of a corrupt capitalist system she has pledged to fight as president.

On Thursday, Warren mocked Buttigieg for a fundraiser held in an elaborate “wine cave” — a phrase that quickly trended on Twitter. Sanders got into the act as well, with his campaign posting a URL, “peteswinecave,” linking to a donation page for his campaign.

Minnesota’s Klobuchar, who is serving her third six-year term in the Senate, went after what might be Buttigieg’s greatest political vulnerability: his lack of statewide or national experience.

The Navy veteran is finishing up his second four-year term as mayor of South Bend. He retorted that he won his mayoral re-election as “a gay dude” in conservative Indiana where Vice President Mike Pence previously served as governor.

It remains to be seen whether Warren or Klobuchar will see a boost in the polls on a night when many viewers may have stayed away because of the holiday season or fatigue over watching coverage of Trump’s impeachment this week.

In earlier Democratic debates, conflict has not proven to go over well with voters. U.S. Senators Cory Booker of New Jersey and Kamala Harris of California and former Housing secretary Julian Castro of Texas all seemed to suffer when they turned aggressive.

Harris has left the campaign trail. And neither Booker nor Castro qualified for the stage on Thursday.