LOS ANGELES (Reuters) - U.S. congressman Seth Moulton, one of 20 Democrats running for president, criticized rivals Bernie Sanders and Elizabeth Warren on Friday, saying they were so liberal they risked handing President Donald Trump a second White House term.

Moulton is a long-shot candidate at this stage. But his comments reflect a growing conflict between the Democratic Party’s moderate and progressive wings that will likely be laid bare during the battle to decide who will take on Republican Trump in next year’s presidential election.

A representative from Massachusetts and Iraq war veteran, Moulton said Trump is a much more difficult candidate to defeat in 2020 than many Democrats realize because of his appeal to voters in the heartland who are frustrated with Washington.

“We can’t go too far left or we will lose middle America,” Moulton said in an interview in Los Angeles, part of a tour to California and other early voting states since he announced his candidacy on Monday.

He said the message of candidates such as Vermont Senator Bernie Sanders and Massachusetts Senator Elizabeth Warren was going to make it difficult to win Congress and “take back the White House.”

While he agreed the wealthy ought to pay their share of taxes, Sanders and Warren wanted to “punish the rich,” Moulton said, which he called un-American.

Moulton, 40, built his political career on challenging the Democratic Party establishment, entering Congress in 2015 after winning a primary challenge against John Tierney, who had held the seat for 18 years.

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After Democrats took control of the House of Representatives in 2018, Moulton helped organize opposition to Representative Nancy Pelosi’s bid to become Speaker for a second time.

In Friday’s interview, Moulton sounded particular alarm over Sanders, a self-described “democratic socialist” elected to the Senate as an independent. Sanders has emerged as an early Democratic front-runner along with former U.S. Vice President Joe Biden, who entered the 2020 field on Thursday.

In some of the harshest words yet uttered by a Democratic presidential candidate against a rival, he said: “Bernie wants to change us into a socialist country, and we’re not a socialist country.”

He added: “That’s not what America is all about. I don’t think that a socialist nominee is going to win the presidency. I’m a Democrat, I’m not a socialist … He’s a socialist, not a Democrat.”

Sanders spokeswoman Sarah Ford responded to the charge by saying the candidate was doing well in the polls because he is a “champion for working people.”

“Senator Sanders has a long and well-known record leading the effort to create a government that works for all Americans,” she told Reuters in an email.

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Warren proposes raising taxes on America’s 75,000 richest families to pay for programs such as universal childcare and universal free public college.

“The problem with some of the candidates in our party is that they’re divisive in the same way that Trump has been so divisive,” Moulton said. “They are pitting different parts of America against each other.”

He said most Americans aspire to be rich. “That’s the sprit of America, that’s the American dream,” he said.

Warren’s campaign did not respond to an email for comment.

Moulton said his candidacy would gain traction by focusing on foreign policy, a subject he said many Democratic rivals were afraid to address, and by running as a “change agent” against the old guard in Washington.