Joseph R. Biden Jr. secured an early advantage on a pivotal night of Democratic primary voting on Tuesday with victories in four Southern states — Virginia, North Carolina, Tennessee and Alabama — demonstrating his strength among African-American voters and dealing a blow to Senator Bernie Sanders and Michael R. Bloomberg.
It was a remarkable show of strength for Mr. Biden, the former vice president, who was reeling after losing the first three nominating states but rebounded with a landslide win in South Carolina on Saturday. But as voters in 14 states and one territory went to the polls on Tuesday, Mr. Biden’s initial success offered only the first indication of the outcome on a day when about a third of the delegates in the Democratic race were at stake.
Mr. Sanders easily carried his home state of Vermont and was locked with Mr. Biden and Senator Elizabeth Warren of Massachusetts in a competitive race in her home state. Mr. Biden’s early lead in the South could well be offset by victories in Western states later: Mr. Sanders was expected to perform strongly in California, the most important prize of the night, where recent polls had shown him with a solid lead over Mr. Biden.
As he did in South Carolina, Mr. Biden rolled to victory in the four Southern states thanks in large part to black voters: More than 60 percent of African-Americans voted for him. Just as worrisome for Mr. Sanders, in Virginia and North Carolina — two states filled with suburbanites — Mr. Biden performed well with a demographic that was crucial to the party’s success in the 2018 midterm elections: college-educated white women.
For his part, Mr. Sanders continued to show strength with the voters that have made up his political base: those under age 40. But his inability to expand his appeal with older voters and African-Americans doomed his candidacy in two swing states where he had aggressively competed, Virginia and North Carolina — just as it did in South Carolina.
The voting followed an extraordinary reframing of the race in the past 48 hours, as moderate candidates came together to form a united front against Mr. Sanders, a democratic socialist whose general election prospects are viewed skeptically by much of the party leadership. Mr. Biden’s overwhelming victory in South Carolina on Saturday established him as the clear front-runner in the Democrats’ centrist wing, prompting two rivals, Senator Amy Klobuchar of Minnesota and former Mayor Pete Buttigieg of South Bend, Ind., to end their bids and throw their support to Mr. Biden.
As Mr. Sanders returned to Vermont, where he voted Tuesday morning, his allies acknowledged that they had been caught off guard by the swiftness with which Mr. Biden’s former adversaries had locked arms to oppose Mr. Sanders’s campaign. They argued that Mr. Sanders was still far better equipped — financially and in his campaign organization — than Mr. Biden to compete for the nomination over a long primary race. And they vowed to highlight to voters the sharp differences in their agendas.
Mr. Biden spent the day in California, kicking off the morning at an upscale diner in Oakland before heading to Los Angeles to campaign and then attend a watch party. In his stump speech, Mr. Biden has taken to routinely hitting Mr. Sanders, arguing that the American people are more interested in results than in a “revolution.”
The two other major candidates, Senator Elizabeth Warren of Massachusetts and Mr. Bloomberg — who was appearing in his first primaries after skipping the contests in the first four states — faced a more daunting challenge. Both have either hinted or stated explicitly in recent days that the race may have shifted so much that their own chances of victory depend largely on the possibility of a contested convention.
Mr. Bloomberg, in Miami, repeatedly flashed irritation at questions about his political standing: Asked by one reporter whether he was siphoning votes from Mr. Biden and helping Mr. Sanders, Mr. Bloomberg countered that Mr. Biden was the one who was taking votes from him. Prodded about whether he could accept finishing third, Mr. Bloomberg replied: “There are only three candidates. You can’t do worse than that.”
When he was reminded of Ms. Warren’s presence in the race, he bristled: “I didn’t realize she’s still in. Is she?”
In the two general election swing states that were called early, Virginia and North Carolina, Mr. Biden’s easy victories illustrated why he had emerged as a singular moderate alternative to Mr. Sanders, who was unable to match Mr. Biden’s strength among African-American and suburban voters.
For Mr. Bloomberg, the results in Virginia and North Carolina were ominous because of the gaping spending gap: While he poured tens of millions of dollars into each state, Mr. Biden scarcely even advertised there, going on the airwaves only last week.
With their long tradition of elevating moderate Democrats, the two states were fertile terrain for Mr. Biden. He got a lift from his triumph in nearby South Carolina, a state many Super Tuesday voters were watching to see whether Mr. Biden could recover from his early struggles, and then won a series of endorsements from party leaders, including many in Virginia.
His advantage only grew after Ms. Klobuchar and Mr. Buttigieg, who were competing for some of the same voters, withdrew from the race and gave their support to Mr. Biden.
Mr. Bloomberg had poured millions of dollars from his personal fortune into TV ads in the two states and made repeated appearances in both, with his advisers holding out the most hope in North Carolina. And while he has been more focused on liberal states, Mr. Sanders made multiple stops in Virginia and North Carolina in the weeks after the New Hampshire primary last month.
The Super Tuesday contests were set to provide the first test of the Democratic presidential candidates on a truly national scale, gauging their appeal to a wide array of constituencies all at once.
Unlike most of the small states that held primaries and caucuses one by one in February, the simultaneous contests on Tuesday represented a diverse cross-section of the communities and constituencies that make up the American electorate.
There was a possibility — though perhaps not a likelihood — that they would render a decisive judgment on the race. Mr. Sanders had hopes, going into Tuesday, that he would be able to amass an insurmountable lead in the delegate count by running up wide margins in several states, most of all in California. Mr. Biden was aiming to block that outcome, and to do a good bit more than that, by uniting moderate forces in the Democratic Party under one banner for the first time in the race.
The Democratic campaign barreled into Super Tuesday in a state of extraordinary flux, as a loose alliance of party leaders, elected officials and centrist voting blocs have seemed to fall in behind Mr. Biden since his weekend triumph in South Carolina. On Monday night, Mr. Biden made appearances in Texas with three former rivals — Ms. Klobuchar, Mr. Buttigieg and former Representative Beto O’Rourke — who are now supporting his candidacy, while Mr. Sanders rallied supporters in Minnesota in an effort to capture Ms. Klobuchar’s home state.
Few presidential candidates have endured the political roller coaster Mr. Biden has found himself riding in recent weeks. After finishing a distant fourth in Iowa and then coming in fifth in New Hampshire, he was short on money, in danger of losing support to Mr. Bloomberg and facing a do-or-die primary in South Carolina.
Yet after shaking up his campaign and installing a longtime adviser, Anita Dunn, as his chief strategist, Mr. Biden was able to claw back into contention by finishing second in Nevada. Then, after two solid debate performances during which his ascendant rivals were the ones under attack, he picked up a crucial endorsement: Representative James E. Clyburn, the highest-ranking African-American in Congress and the most influential South Carolina Democrat, came out for Mr. Biden at an emotional news conference.
With Mr. Clyburn’s imprimatur, Mr. Biden built a considerable advantage with black voters that propelled him to a 28-point rout in South Carolina.
His big victory immediately prompted what Mr. Biden had been seeking for months: a flood of support from Democratic leaders, donors and his onetime rivals. By Monday night, just 48 hours after what was his first victory over three separate presidential runs, Mr. Biden was at the rally in Dallas, gaining the endorsements of his three former opponents.
Mr. Sanders entered the day with a modest delegate advantage and was poised to extend his lead because, having consolidated much of the party’s left, he was better organized and better funded than any of the more moderate candidates.
Having finished at the top of Iowa and New Hampshire with less than 30 percent of the vote, Mr. Sanders extended his lead with a commanding victory in Nevada, where he demonstrated that he could expand his heavily white base to include Hispanic voters.
While he struggled with South Carolina’s majority-black electorate, Mr. Sanders went into Tuesday well positioned to capture most of the delegates in Vermont, Massachusetts, Minnesota, Colorado, Texas and, perhaps most important, California.
And after training his fire primarily at Mr. Bloomberg, Mr. Sanders is shifting to take on Mr. Biden. His campaign released a scorching online video citing Mr. Biden’s vote to authorize the use of force in Iraq and linking the former vice president to the Bush administration officials who oversaw the war there. At a rally on Monday night in St. Paul, Minn., the senator said that Mr. Biden was “wrong on the issues” and “wrong with regard to his vision for the future,” and he argued that Mr. Biden’s past support for free trade agreements would make it difficult for him to win in Midwestern states.
The propulsive energy behind both Mr. Biden and Mr. Sanders posed a challenge to two other formidable candidates, Ms. Warren and Mr. Bloomberg, who have charted drastically different paths to the final stages of the Democratic race. For Mr. Bloomberg, the Super Tuesday contests were the first time he appeared on a ballot, because he bypassed the February contests entirely.
Mr. Bloomberg, the wealthy former mayor of New York City, has been a pervasive presence in the race: He has run more than half a billion dollars in paid advertising since he announced his campaign in November, offering himself to the Democratic establishment as a potential savior if Mr. Biden failed to halt Mr. Sanders in the earliest primaries and caucuses.
Yet Mr. Bloomberg’s prospects were uncertain heading into Super Tuesday, after his disastrous performance in a debate last month in Las Vegas and Mr. Biden’s resurgence in the polls.
Mr. Bloomberg’s campaign publicly signaled he planned to forge ahead even if he faced a disappointing night, announcing plans to stump later this week in Pennsylvania and Michigan, and holding his election-night event in Florida, which holds a primary in two weeks. But advisers to Mr. Bloomberg have also acknowledged that the former mayor would make a cold assessment of his options once the results of Super Tuesday became known. He is likely to face intense pressure to make way for Mr. Biden if his self-funded candidacy does not yield impressive results.
Ms. Warren could face pressure of a different kind, though she is seen as less likely than Mr. Bloomberg to bow to the results on Tuesday. With replenished financial coffers, she has vowed to press ahead in the race and to pick up as many delegates as possible, on the way to what her advisers believe will be a contested convention in Milwaukee this summer.
Katie Glueck contributed reporting from Oakland, Calif., and Patricia Mazzei from Miami.