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.