Rep. Seth Moulton Seth MoultonThe Hill's Morning Report - Presented by Facebook - Markey defeats Kennedy; Trump lauds America's enforcers in Wisconsin Moulton fends off primary challenges in Massachusetts Portland: The Pentagon should step up or pipe down MORE (D-Mass.), who announced his presidential campaign this week, went after fellow 2020 contenders Sens. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.), saying the progressive lawmakers were too liberal to beat President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE.

The long-shot candidate cautioned that Democrats may underestimate Trump’s appeal in the nation’s heartland where many voters are frustrated with Washington politics.

“We can’t go too far left or we will lose middle America,” Moulton told Reuters in an interview during a campaign tour of California.

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Moulton, a self-proclaimed moderate, told the outlet that Warren's and Sanders's message would make it difficult to win Congress and “take back the White House” in the next election cycle.

Moulton said he agreed with the two senators' idea that wealthy Americans should pay higher taxes, but denounced Sanders and Warren for endorsing policies that he said intend to “punish the rich.”

Both Sanders and Warren have put economic issues at the center of their campaigns, railing against gains made by Wall Street and the top-earning Americans as evidence that the economy is stacked against the working class.

Moulton called such rhetoric and related policies, including Warren’s plan to tax America’s 75,000 richest families to pay for social programs, as “divisive.”

Moulton focused much of his fire on Sanders, slamming the Vermont senator who has emerged as an early frontrunner in the primary field.

“Bernie wants to change us into a socialist country, and we’re not a socialist country,” he said.

“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’s campaign told Reuters that the senator is yielding strong poll figures and fundraising numbers 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,” Sanders spokeswoman Sarah Ford told Reuters in an email.

Warren’s campaign did not immediately respond to requests for comment from The Hill or Reuters.

Moulton's comments come as the Democratic Party grapples with whether it should throw its support behind a moderate candidate who can win disaffected Republicans and independents or a progressive who can excite the base.