DETROIT — At Tuesday night’s debate, the moderate Democrats delivered a chorus of complaints, to little apparent effect:

They argued single-payer health care would never work in America.

They insisted that decriminalizing border crossings would be politically toxic.

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They accused the liberals of misunderstanding the economy and overlooking the essential conservatism of big parts of the country.

But Senators Bernie Sanders and Elizabeth Warren gave no ground, countering that their centrist critics — men like Gov. Steve Bullock of Montana and former Representative John Delaney of Maryland — lacked both the ambition to fight for policies that would help voters the most, and the grit to win a difficult election against President Trump.

If a moderate candidate is going to rise from the pack of 2020 Democratic presidential hopefuls, no one in the forum appeared to gain enough political propulsion to catch up with Mr. Sanders and Ms. Warren. The two populist liberals have defined the Democratic Party’s policy debate up to this point, a dynamic that only intensified on Tuesday.