ST. PAUL, Minn. — On Sunday night, Faiz Shakir, the campaign manager for Senator Bernie Sanders, received a call from his onetime boss and longtime mentor, former Senator Harry Reid of Nevada.
Mr. Reid was delivering disappointing news: He planned to endorse former Vice President Joseph R. Biden, Jr., who was quickly emerging as Mr. Sanders’s chief rival in the presidential primary.
The call would set off a whirlwind 24 hours for the Sanders campaign, which just days earlier had seemed as if it was on a runaway train to the Democratic nomination. Hours after Mr. Reid’s phone call, as Mr. Sanders prepared to host a rally in Salt Lake City, news broke that Senator Amy Klobuchar of Minnesota, a moderate, planned to drop out of the race and endorse Mr. Biden. Reports also flowed in that Pete Buttigieg, the former mayor of South Bend, Ind., who had dropped out Sunday night, would back Mr. Biden, too.
The swiftness of the coalescence around Mr. Biden caught the Sanders team off guard. Even after Mr. Biden handily won in South Carolina on Saturday, beating Mr. Sanders by nearly 30 percentage points, aides had spent Sunday reluctant to declare the primary a two-person race between Mr. Sanders and Mr. Biden. By Monday afternoon, however, everyone on the campaign recognized the new dynamics.
“We always anticipated that there would be consolidation of an establishment side,” Mr. Shakir said in an interview Monday night. “It’s one thing to know it’s going to happen, and it’s another thing to watch it happen so very quickly.”
“Because of the swiftness with which it moved,” he added, “it’s becoming clear that in order for us to win this nomination, that road clearly flows through Joe Biden.”
As the contours of the race heading into Super Tuesday became more apparent, the campaign began reaching out to progressive groups and politicians, Mr. Shakir said, hoping to consolidate more support quickly. It also hashed out a strategy that involved drawing more explicit contrasts to Mr. Biden.
The campaign plans to be on the air with advertisements in key states that will vote on March 10 and March 17, including Florida, Michigan, Ohio and Arizona; the ad blitz will include a spot about Mr. Biden’s record on Social Security. It is considering running an ad in Midwestern states like Michigan and Ohio — where both candidates are competing for an overlapping slice of white working-class voters — that will emphasize Mr. Biden’s record on trade, including his support for the North American Free Trade Agreement.
The Sanders campaign also circulated a memo to surrogates and supporters highlighting “the contrasts that we expect to now be in the spotlight as we head into Super Tuesday and beyond.”
“We are now entering the phase of the primary in which the differences between Bernie and Biden will take center stage,” the memo said. “These differences make clear that the choice between these two candidates is stark.”
Since Mr. Sanders got into the race a year ago, his top advisers have ached for a two-way race with Mr. Biden, viewing the moderate former vice president as the perfect foil for Mr. Sanders’s promise of a political revolution: a throwback Democrat who touts his ability to work with Republicans and who has been criticized by rivals for telling donors that under a Biden administration, “nothing would fundamentally change.”
But the reality of such a direct confrontation with Mr. Biden, a five-decade fixture in Washington politics, poses challenges for Mr. Sanders, whose style of grass-roots politics and long history as an outsider means he cannot muster the same kind of institutional forces. Instead, he relies on a loyal army of individual donors who give $18 at a time, and a progressive network that for all of its ambition remains in some ways disjointed and uncoordinated.
“There’s a reason the establishment has power, keeps power and maintains powers, because these are the things that they do well,” Mr. Shakir said.
Complicating matters, Mr. Sanders’s chief ideological rival, Senator Elizabeth Warren of Massachusetts, remains in the race despite finishing out of the top two in all of the first four nominating contests — including a distant fifth in South Carolina. Though there have been fissures in the united facade their two campaigns presented for much of the primary race, Sanders aides respect her and do not want to publicly pressure her to drop out.
“We respect the fact that she’s going to make whatever decision she makes, and she should be allowed to do that,” Mr. Shakir said, adding that she “should be given the time and space” to determine her path forward.
He declined to say whether the two campaigns had discussed efforts to consolidate the liberal wing of the party.
Even before Mr. Reid’s phone call, the Sanders campaign had been gearing up for a showdown with Mr. Biden. On Sunday, it announced it would be up on the air in nine more states this week. In the days leading up to Tuesday, Mr. Sanders went on a frenetic, exhausting swing of states that at times left him hoarse, holding especially enormous rallies in Los Angeles, Boston and northern Virginia.
Speaking to reporters Monday morning during a rally in Salt Lake City, Ari Rabin-Havt, a deputy campaign manager, struck a confident tone, saying that the campaign was not nervous about any of the recent developments and that it did not intend to change its strategy.
“Watching the campaign, watching the 10 debates unfold, we believe they have constantly shown that Bernie is the strongest candidate to defeat Donald Trump, and that’s still the case,” Mr. Rabin-Havt said. “And we think we still are in a very strong position heading into Super Tuesday.”
During that rally, Mr. Sanders made no mention of Ms. Klobuchar’s decision to drop out of the race and endorse Mr. Biden, nor did he talk about Mr. Reid.
But earlier in the day he told reporters: “We are taking on the establishment. And I fully understand, no great surprise to me, that establishment politicians are not going to endorse us.”
As the day wore on, however, the campaign realized the scale of the effort to block Mr. Sanders’s path to the nomination. Mr. Shakir blasted out a tweet calling Mr. Reid’s endorsement “disappointing,” in what became something of a call to arms for Sanders supporters.
By Monday night — when Ms. Klobuchar, Mr. Buttigieg and former Representative Beto O’Rourke were joining Mr. Biden in Texas for a public show of support — the Sanders campaign began to put some of its new strategy into play, and the candidate himself displayed more public urgency.
At a rally in St. Paul, Minn., before thousands of supporters, he laced into Mr. Biden for his record on Social Security, bankruptcy and his vote to authorize the war in Iraq.
“Joe is a decent guy,” Mr. Sanders said, as the crowd began to boo. “He’s just wrong on the issues. He’s just wrong with regard to his vision for the future.”
He reserved his most cutting remarks for Mr. Biden’s record on trade.
“Here in the Midwest and all across this country, we have lost millions of good paying manufacturing jobs because of disastrous trade agreements,” Mr. Sanders said, his target firmly in sight. “Joe Biden voted for those trade agreements. Does anybody think that Joe can go to Michigan or Wisconsin or Indiana or Minnesota, and say, ‘Vote for me, I voted for those terrible trade agreements’? I don’t think so.”
And in a flash of magnanimity Mr. Sanders encouraged Buttigieg and Klobuchar supporters to join him instead of Mr. Biden.
“The door is open,” he said, grinning. “Come on in!”