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.
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.
And so on Wednesday, many moderate Democrats were turning their eyes once more to Joseph R. Biden Jr. The hopes of jittery moderates rest on him, now more than ever, and Mr. Biden’s ability to dominate tonight’s debate may settle decisively whether or not his wing of the party will have a compelling champion as it enters a six-week stretch until the next debate in September.
Alone among the top candidates of the Democratic race, Mr. Biden has positioned himself as a proud man of the center-left, an advocate for modest change with a keen sensitivity to voters’ anxieties about drastic reform and the confining sloth of the country’s political institutions. Unlike other moderate Democrats, Mr. Biden needs no moment of debate-stage electricity to introduce himself to the country, no sudden rise in the polls to become relevant in a race he already leads.
What he does need, however, is a display of vision and force. In the first round of debates in June, Mr. Biden confounded his supporters — and the Democratic Party’s relatively moderate establishment — by largely declining to make an affirmative case for his ideas. He spent the evening on defense over his decades-long record on matters of race, a subject that could thwart him again on Wednesday, when he will once more appear onstage beside Senator Kamala Harris of California.
Should Mr. Biden stumble badly again, it could open the way for relatively low-profile moderates to start picking apart Mr. Biden’s electoral coalition.
Candidates like Senator Amy Klobuchar of Minnesota and Mayor Pete Buttigieg of South Bend, Ind., could move aggressively on his supporters in the Midwest, while the two black candidates in the race, Ms. Harris and Senator Cory Booker of New Jersey, could suddenly see a wider path to victory in the Southern states where most black voters currently support Mr. Biden.
But there is no candidate in the race who seems poised to inherit Mr. Biden’s entire coalition if he collapses.
Up to this point, moderate Democrats acknowledge, the race has seemed to lack a leading candidate capable of making an uplifting case for less-populist ideas — a moderate who could inspire voters, not just scold the left.
Important officials in some of the key swing states have indicated they are impatient for a figure like that to emerge.
“Michiganders are practical people, so they want to hear practical ideas of how to fix things, not pie-in-the-sky, over-the-top structural change in our economy and in our country,” said Representative Elissa Slotkin, a moderate Democrat elected last November, in an interview on Tuesday afternoon.
Ms. Slotkin said voters were looking for a unifying candidate who projected “strength of purpose,” not merely a partisan critic of Mr. Trump.
Complicating matters for moderate Democrats is the sense among liberals that their party has tried a comparatively cautious approach too many times, with little to show for it. Hillary Clinton struggled to unite the left and the center in 2016, both in the primary and the general election, while earlier center-left nominees, like Al Gore in 2000 and John Kerry in 2004, fell even further short of that goal. Much of the party is now open to the idea that defensive politics is a loser’s game, and the Sanders-Warren appeals to boldness on Tuesday resonate more powerfully than in the past.
Only a few candidates in recent decades have managed to project a moderate vision of change that still excited much of the Democratic base, and in this century the only candidate fitting that description has been Barack Obama.
The number of candidates who might have the chance to mimic that role — a happy warrior from the political center — might dwindle quickly after Wednesday’s debate. No candidate besides Mr. Biden who is running explicitly as a centrist has qualified yet for the next round of debates in September, though Ms. Klobuchar is said to be close to that threshold.
Mr. Bullock of Montana, for instance, who made a hearty case on Tuesday against the agenda of Ms. Warren and Mr. Sanders, would have to gain quickly in the polls and amass many more campaign donors in order to carry that fight forward into the fall.
In a scenario where Mr. Biden fades and other avowed centrists fail to qualify for the next debate, that could lead moderate voters to take a closer look at a number of liberals who are somewhat less populist and ideological in their instincts than Mr. Sanders and Ms. Warren — people like Mr. Buttigieg or even former Representative Beto O’Rourke of Texas, two young men who have escaped sharp ideological definition, appealing to the left without quite endorsing policies like “Medicare for All.”
Mr. Biden, however, appears determined to reassure Democrats that he can wage that fight on his own. On Wednesday morning, he published a column on CNN.com drawing a bright contrast with the left on health care policy, and seeking to cast his more incremental position — he favors creating an optional government plan, not “Medicare for all” — as a matter not just of political caution but also principled concern for voters in distress.
“Time is precious,” Mr. Biden wrote, “and families experiencing health care problems and rising medical bills right now can’t wait for us to start over with a complete overhaul of our health care system.”
In talking points circulated to Mr. Biden’s allies on Tuesday evening, after the debate that centered on Mr. Sanders and Ms. Warren, his campaign indicated Mr. Biden was eager to “draw contrast where there are policy disagreements in the field,” most of all on health care. Mr. Biden, the document promised, would use the debate to “demonstrate how we’ll defeat Donald Trump and move the country forward.”
But the document also acknowledged the former vice president might be in for a rough evening, warning that “every candidate’s goal for the second debate is to knock out Biden.”
And while Mr. Biden has projected a fighting spirit in recent days, maintaining that confident posture on the debate stage is another matter entirely.