MANCHESTER, N.H. — The Democratic presidential primary is entering an intensely tumultuous phase, after two early contests that have left former Vice President Joseph R. Biden Jr. reeling and elevated Senator Bernie Sanders but failed to make any candidate a dominant force in the battle for the party’s nomination.
Within the Democratic establishment, the results have deepened a mood of anxiety and frustration: The collapse of Mr. Biden’s support in the first two states, and the fragmentation of moderate voters among several other candidates, allowed Mr. Sanders, a Vermont progressive, to claim a victory in New Hampshire and a split decision in Iowa with former Mayor Pete Buttigieg of South Bend, Ind.
In both states, a majority of voters supported candidates closer to the political center and named defeating President Trump as their top priority, but there was no overwhelming favorite among those voters as to which moderate was the best alternative to Mr. Sanders. Unless such a favorite soon emerges, party leaders may increasingly look to Michael R. Bloomberg as a potential savior.
In an unmistakable sign of Mr. Bloomberg’s growing strength and Mr. Biden’s decline, three black members of Congress endorsed the former mayor of New York City on Wednesday, including Representative Lucy McBath of Georgia, a high-profile lawmaker and gun-control champion in her first term — and a senior adviser to Mr. Bloomberg told campaign staff that internal polling showed the former mayor now tied with Mr. Biden among African-Americans in March primary states.
The turmoil in the party has the potential to extend the primary season, exacerbating internal divisions and putting off the headache of uniting for the general election for months.
The Democrats’ proportional system of allocating delegates could make it all but impossible to avert such an outcome. With no winner-take-all contests, and no indication yet that Mr. Sanders can broaden his appeal or that a moderate can coalesce support, the candidates are poised to keep splitting delegates three or four ways, as they did in Iowa and New Hampshire.
“We are obviously going to have a longer battle here,” said Mark Mellman, a Democratic pollster who directed an anti-Sanders ad campaign in Iowa.
The leading candidates are plainly worried about the party’s divisions, and signaled as much in their speeches in New Hampshire on primary night: Mr. Sanders, blamed by much of the party for his slashing approach to the 2016 primaries, stressed in his victory speech that the most important task was defeating Mr. Trump, while Mr. Buttigieg urged his supporters to “vote blue, no matter who” in November.
At the moment, no one is close to being the last candidate standing. But unless another Democrat rapidly consolidates support, Mr. Sanders could continue to win primaries and caucuses without broadening his political appeal, purely on the strength of his rock-solid base on the left — a prospect that alarms Democratic Party leaders who view Mr. Sanders and his slogan of democratic socialism as wildly risky bets in a general election.
The Biden team stoked that sense of alarm on Wednesday: Representative Cedric Richmond of Louisiana, a co-chairman of Mr. Biden’s national campaign and a former chairman of the Congressional Black Caucus, warned on a conference call with reporters that Democrats would risk “down-ballot carnage” if they selected Mr. Sanders.
“If Bernie Sanders was at the top of the ticket, we would be in jeopardy of losing the House,” Mr. Richmond said. “We would not get the Senate back.”
Yet in a reflection of the multidimensional melee that allowed Mr. Sanders to claim victory in New Hampshire with the smallest plurality of any winner in decades, Mr. Richmond also criticized two other candidates, Mr. Bloomberg and Mr. Buttigieg, lumping them into the same risky group and arguing that Democrats should not “take a chance with a self-defined socialist, a mayor of a very small city, a billionaire who all of a sudden is a Democrat.”
Mr. Mellman said Mr. Sanders would continue to benefit as long as there was a relative abundance of moderate candidates in the race. “The longer more of those people stay in,” he said, “the easier it is for Sanders to skate through.”
There is no sign that any of the half-dozen major candidates left in the race are headed for the exits: Mr. Buttigieg and Mr. Biden will have to contend in the Nevada caucuses against Senator Amy Klobuchar of Minnesota, who finished a strong third in New Hampshire, while on the left Mr. Sanders still faces a dogged competitor in Ms. Warren. Unless one candidate comes out of Nevada and South Carolina with a powerful upper hand, it is quite likely that the same atomized delegate count could continue into Super Tuesday, when 15 states and territories, accounting for nearly 40 percent of all delegates in the Democratic race, cast ballots on March 3.
Indeed, with early voting already taking place in California and other Super Tuesday states, and no dominant front-runner, the fragmentation may already be well underway.
In Arkansas, a Super Tuesday state where early voting starts next week, a poll taken after Iowa illustrated the Democrats’ dilemma: Mr. Bloomberg, Mr. Biden, Mr. Sanders and Mr. Buttigieg were each winning 16 to 20 percent of the vote.
All of those candidates are increasingly confronting Mr. Bloomberg’s presence as a rival in the March primaries. Mr. Bloomberg skipped all four February contests but has climbed into double digits in national polls on the strength of an enormous and sustained advertising campaign, funded from his personal fortune.
On a conference call with campaign staff members on Wednesday afternoon, Howard Wolfson, Mr. Bloomberg’s senior adviser, said that internal tracking data showed that the former mayor had pulled “very narrowly” into first place across the March primary states, inching ahead of Mr. Sanders over all and tying Mr. Biden among African-American voters.
Though Mr. Wolfson did not provide specific numbers, he said Mr. Biden had “rather precipitously fallen” in the larger array of states voting next month, according to Bloomberg polling.
But Mr. Bloomberg is facing new tests as a candidate: For the first time, he may qualify for a televised debate, next week in Las Vegas, and he has come under newly direct criticism from other Democrats for his record on policing and much else.
Mr. Wolfson acknowledged as much on the conference call, telling staff members that Mr. Bloomberg would have a “bigger target on his back” as his numbers rose. He said Mr. Bloomberg would address scrutiny of his support for stop-and-frisk policing by calling it “the biggest regret of his 12 years as mayor,” and saying that the language he had used in the past to defend it did not “reflect who he is or what is in his heart.”
But the recently circulated audio recording of Mr. Bloomberg in 2015 matter-of-factly stating that “the real crime is” almost always committed by young “male minorities” quickly ricocheted across the tight-knit community of black political leaders.
J. Todd Rutherford, the minority leader of the South Carolina House, said many African-Americans had increasingly recognized that Mr. Biden did not have “what it takes” and had been ready to bolt to the former New York mayor.
“A lot of people would’ve said Bloomberg last week, but now I don’t know,” said Mr. Rutherford, alluding to the recording and declaring that the gnawing uncertainty hanging over the Democratic race was “really scary.”
Mr. Biden and other candidates have indicated that they intend to challenge Mr. Bloomberg on race in the coming days, and his resiliency, or lack thereof, on the subject could shape the primary campaign.
Yet even supporters of Mr. Biden acknowledge that if one of the moderates doesn’t take a clear lead with that faction of the party after Nevada, the eyes of many establishment-aligned Democrats will turn to Manhattan.
“The longer the waters are muddy, the better off Bloomberg is,” said former Gov. Jim Hodges of South Carolina, who recently backed Mr. Biden.
The campaign in Nevada is as disordered as anything else in the Democratic race, according to people closely watching the contest there. But as in New Hampshire, Mr. Biden long held a considerable advantage as the candidate perceived as the safe and electable choice, while Mr. Sanders entered the race with a strong bloc carried over from his last run for the presidency. It remains to be seen whether Mr. Biden will bleed support there as rapidly as he did in New Hampshire, or whether any other candidate will be able to take advantage of his fall.
Tick Segerblom, a prominent Sanders backer in the state who is a member of the Clark County Commission, said Mr. Biden’s national plunge would upend the campaign in Nevada. He said that Mr. Sanders could count his “25 percent,” but that his ability to expand his coalition was an open question.
“Bernie is still alive and Biden is definitely a disaster,” Mr. Segerblom said. “I think Pete is going to do very well — he’ll be able to pick up the Biden people.”
Representative Dina Titus, perhaps the most prominent Biden supporter in the state, said the campaign needed to send in the political cavalry to stave off defeat.
“He could certainly use more hands — and they’re supposedly coming now,” she said.
She added that she would spend her time helping Mr. Biden with senior groups and labor unions. But the former vice president’s hope that the most influential union in the state, the Culinary Workers, would endorse him in an effort to halt Mr. Sanders has been dimmed after his poor performance in the first two states.
In South Carolina, even moderate Democrats who are sympathetic to Mr. Biden believe he’s in grave danger of losing the state.
“He’s wounded,” Tyler Jones, a Charleston-based Democratic strategist, said of the former vice president. Like other political professionals in the state, Mr. Jones is increasingly less concerned about Mr. Biden’s weakness than about the billionaire Tom Steyer’s strength — and what it means for the nominating process.
Mr. Steyer has been pouring money into South Carolina, cutting into Mr. Biden’s lead with black voters and raising the specter of Mr. Sanders’s winning another state with a plurality thanks to a divided electorate.
“A vote for Steyer is a vote for Bernie, which is a vote for Trump,” said Mr. Jones, who believes Mr. Sanders cannot win the general election and wants to stop his campaign “dead in its tracks.” But he acknowledged that urging voters to act strategically and reject Mr. Steyer was easier in theory than in execution.
Other Democrats are turning to what they believe is a more simple solution.
In early January, Representative Gregory Meeks of New York offered an off-the-cuff assessment of the Democratic race: Should Mr. Biden wheeze in the early states, many in the party would turn to Mr. Bloomberg as a Plan B.
“If Mr. Biden can’t get out of New Hampshire and Iowa, then Bloomberg has Super Tuesday,” Mr. Meeks said at the time.
On Wednesday, he was one of the three black lawmakers who endorsed Mr. Bloomberg.
Thomas Kaplan contributed reporting.