HOUSTON — Energized by his landslide victory in the Nevada caucuses, Senator Bernie Sanders turned his focus to President Trump on Sunday while his campaign made plans to try to win the coming South Carolina primary and amass an insurmountable delegate lead on Super Tuesday next week.
Mr. Sanders plans to be up on the air with commercials in every South Carolina media market this week, and his staff is scrambling to add new rallies to his schedule as they take aim at their next big target: overtaking the front-runner in Saturday’s primary there, former Vice President Joseph R. Biden Jr., to all but extinguish his candidacy.
But attacks on Mr. Sanders began mounting on Sunday, with Mr. Biden criticizing him as disloyal to former President Barack Obama — a charged message with the predominantly black electorate in South Carolina — and others describing the Vermont senator as a long shot against Mr. Trump. Mr. Sanders, in turn, used a rally in the Super Tuesday state of Texas to highlight some favorable polling numbers against the president — and attempt to reassure Democrats about his electability if he wins the nomination.
With seven other candidates competing against the liberal Mr. Sanders, raising the possibility of a fractured moderate vote in South Carolina and on Super Tuesday, pressure is also starting to grow on some Democrats to drop out — particularly Senator Amy Klobuchar, the billionaire Tom Steyer and even former Mayor Pete Buttigieg, who narrowly won the Iowa caucuses. None has shown an ability to build a diverse coalition of voters, which would be crucial to beating Mr. Sanders, yet none showed signs of quitting anytime soon.
The Democratic race is now entering a critical nine-day stretch that could effectively determine the nomination, with the future of multiple candidates riding in part on the next televised debate on Tuesday night, the South Carolina vote and the Super Tuesday contests.
There were new twists in the race on Sunday, such as Mr. Steyer qualifying to join in the debate and an announcement by Representative James E. Clyburn, the most influential Democrat in South Carolina, that he will endorse a candidate on Wednesday. Adding to the unpredictability is former Mayor Michael R. Bloomberg, who will try to start recovering from his disastrous debate performance last week by appearing for the first time in a CNN town hall, on Wednesday, as well Tuesday’s debate.
Aides to Mr. Sanders are racing to consolidate his political advantages to help him withstand any winnowing of the field, given that perhaps the biggest threat he now faces are anti-Sanders voters coalescing around one alternative candidate.
He enters this next phase of the race with a pair of enviable assets. After finishing near the top in Iowa, winning New Hampshire and then overwhelming his rivals in Nevada, the first state with a racially diverse electorate, he has the aura of success — and winning usually begets winning in presidential primaries.
At the same time, the anti-Sanders camp remains split: Mr. Biden, Senator Elizabeth Warren, Mr. Buttigieg and Ms. Klobuchar are all insisting on staying in the race while Mr. Bloomberg is spending heavily on advertising in Super Tuesday states like Texas and California, where Mr. Sanders could build up a delegate lead if the remaining vote is divided.
In the aftermath of Mr. Sanders’s triumph in Nevada, the question that increasingly looms over the race is whether Super Tuesday, when 16 contests are held on March 3, will be the day he puts himself on a glide path to the nomination.
On Sunday, his aides began taking steps toward trying to do just that.
After months of downplaying Mr. Sanders’s prospects in South Carolina, where he lost badly in the 2016 primary because of his weakness with black voters then, his campaign was blunt about its objective in Saturday’s vote.
“We’re fighting to win in South Carolina,” said Jeff Weaver, a top Sanders strategist, noting that polls show them “moving up considerably on the vice president.”
Should Mr. Sanders defeat or even finish near Mr. Biden in South Carolina, a state the former vice president has long seen as his firewall, it could vault him into Super Tuesday three days later with such force that it may be difficult for any of his opponents to catch up with him.
That’s when Mr. Sanders is hoping to pile up delegates while also inflicting symbolic blows on his opponents by winning in some of their home states, including Ms. Warren’s Massachusetts and Ms. Klobuchar’s Minnesota, both of which Mr. Sanders plans to visit before March 3, according to aides.
Yet it is Mr. Sanders’s decision to compete aggressively in South Carolina, where he has long lowered expectations by vowing only to “do well” there, that illustrates how rapidly this race is moving to him. More than half of the state’s Democratic electorate is expected to be African-American, one of Mr. Biden’s strongest blocs of support, and the former vice president has led in the polls there since the outset of the race.
Yet as Mr. Sanders made clear on Sunday, the primary has taken a turn.
Addressing over 6,000 supporters at the University of Houston for an afternoon rally, he largely ignored his Democratic rivals and boasted about his lead over Mr. Trump in some head-to-head polls.
“Some of the folks in the corporate media are getting a little bit nervous and they say, ‘Bernie can’t beat Trump,’ so let’s look at some of the polls out today,” said a notably upbeat Mr. Sanders, before rattling off numbers that showed him beating Mr. Trump nationally and in battleground states.
Even as Mr. Sanders braced for an onslaught of attacks many in his orbit thought would have come by now — “there are a lot of knives out for Bernie Sanders,” Mr. Weaver said — the senator appeared to be relishing his back-to-back victories. He and his wife and close political confidante, Jane, had a glass of wine on his charter flight Saturday evening after the Nevada results came in, according to a Sanders aide. By Sunday morning, though, Mr. Sanders’s Democratic opponents were ready to try to spoil the moment.
Speaking to reporters after attending an African-American church service in North Charleston, S.C., Mr. Biden acknowledged that Mr. Sanders “does have momentum” before seeking to deflate his candidacy.
The former vice president suggested that Mr. Sanders, an avowed democratic socialist, could hurt Democrats’ chances of maintaining control of the House in 2020 and winning back the Senate. And he sought to undermine his opponent’s chances to win black voters by boasting that he’s had Mr. Obama’s “back the whole time” while saying Mr. Sanders weighed a primary bid against him in the 2012 election. (The Sanders campaign has denied that he considered challenging Mr. Obama.)
In an interview on MSNBC on Sunday night, Mr. Biden pressed the point further, portraying Mr. Sanders as a foe of the Obama administration who could have cost Mr. Obama a second term.
“We’ve just had significant disagreements with Bernie, but the way he talks about it, you’d think he and Barack were close buddies,” Mr. Biden said. He added, “I know they were not.”
While his supporters sought to put the best face on his apparent, and distant, second-place finish in Nevada, Mr. Biden seemed likely to receive some long-awaited good news in South Carolina: the endorsement of Mr. Clyburn, the longtime lawmaker and highest-ranking African-American in Congress.
In an interview, Mr. Clyburn said he would disclose his support on Wednesday, the day after the next debate. He cited his “longtime friendship with Joe,” but otherwise declined to reveal his preference. However, a Democratic official close to the Biden campaign, speaking on the condition of anonymity so as not to reveal internal planning, said they expected to receive Mr. Clyburn’s support.
Mr. Clyburn, appearing on NBC’s “Meet the Press” on Sunday, praised Mr. Sanders on their shared policy priorities like community health centers, but also raised doubts about his chances in South Carolina. “Anybody who refers to themselves as a democratic socialist, that word has always had really dire consequences throughout South Carolina,” Mr. Clyburn said.
For Mr. Biden, a bigger matter than endorsements is the need for some of his rivals to exit the race. Yet each of them believes they have as much of a claim on staying in the primary as Mr. Biden, who finished well behind in the first two states, according to their top advisers.
Speaking to reporters Sunday, Ms. Klobuchar dismissed a question about whether she’d quit the race.
“Why would I get out, that’s not even a close call for me,” she said. “And I think, why would you have a call for the two women to get out when you have two billionaires in the race”
Appearing at campaign events in North Dakota, Virginia and Colorado on Sunday, Ms. Klobuchar, Mr. Buttigieg and Ms. Warren, like Mr. Biden, warned Democrats about the risk of nominating Mr. Sanders.
However, a number of moderate Democrats, some affiliated with Mr. Biden and others uncommitted, said it was Mr. Buttigieg, Ms. Klobuchar and Mr. Steyer who were only helping Mr. Sanders by staying in the race as spoilers.
Some of the officials, like Susan Swecker, the chairwoman of the Virginia Democratic Party, were subtle with their pleadings.
“It would certainly make voter decisions easier because people are still shopping,” said Ms. Swecker, who has not taken sides and whose state votes a week from Tuesday. “If there is less on aisle that makes the choice easier.”
But other Democrats uneasy about the prospect of Mr. Sanders being their standard-bearer were more direct.
“Some folks need to look hard at whether they are viable and contributing to a positive path forward for our party,” said Senator Christopher Coons of Delaware, a Biden supporter. He carefully avoided mentioning names, but said those who remained in the race were undermining the party’s chance for a “shorter and less-divisive primary.”
Bakari Sellers, a former South Carolina legislator who has not endorsed, was happy to name names.
“Pete and Amy have no path because they have no coalition built — they don’t do well with Hispanic or Black voters,” Mr. Sellers said of Mr. Buttigieg and Ms. Klobuchar.
And Mr. Sellers said Mr. Steyer’s candidacy was a “vanity project” that would only siphon away votes from Mr. Biden and thrust Mr. Sanders into contention in South Carolina.
Yet even if Mr. Steyer underperforms in South Carolina, as he did in Nevada despite vastly outspending the field, and Mr. Biden emerges with a clear victory, moderate Democrats are still facing a larger dilemma. Mr. Bloomberg and Mr. Biden are on a collision course to divide votes, particularly among African-Americans, on Super Tuesday.
Sydney Ember reported from Houston and Austin, and Jonathan Martin from Las Vegas. Reporting was contributed by Thomas Kaplan from North Charleston, S.C.; Lisa Lerer from Arlington, Va.; Nick Corasaniti from Fargo, N.D.; and Shane Goldmacher from Denver.