A parade of establishment Democrats began coalescing around Joe Biden on Monday, an attempt to bolster the former vice president and stall Sen. Bernie Sanders’s ascent as voters in 14 states prepared to cast ballots Tuesday, the most consequential day of the presidential nominating contest.

On a day with cascading developments that rapidly recast the presidential race, Sen. Amy Klobuchar (Minn.) dropped out and rushed to join Biden at his rally in Dallas on Monday night. In a visual symbol of Biden’s attempts to consolidate the moderates in the party, former South Bend, Ind., mayor Pete Buttigieg — who dropped out Sunday night and who had an intensely frosty relationship with Klobuchar — also scrambled to get to Texas and endorse Biden.

“We need a politics that’s about decency, a politics that brings back dignity. That’s what Joe Biden has been practicing his entire life,” Buttigieg said at a restaurant in Dallas, ahead of the rally where Klobuchar declared: “I cannot think of a better way to end my campaign than joining his.”

Beto O’Rourke, the former Texas congressman who ended his presidential bid in November, joined Biden on stage at the end of the Dallas rally — and concluded a speech by inviting him for dinner at a nearby Whataburger.

Biden seemed taken aback by the swift change in fortunes. He told Buttigieg that he reminded him of his late son, Beau, the highest compliment he can offer. He told the crowd Klobuchar has a long political future ahead, and he told O’Rourke, whose candidacy was marked by liberal positions on gun control, “You’re going to take care of the gun problem with me. You’re going to be the one who leads this effort.”

It was the second straight day that moderates, previously paralyzed over whom to rally behind, rushed to join Biden’s campaign. Harry M. Reid, a former Senate majority leader from Nevada, endorsed Biden along with other Democrats including Susan E. Rice, a former national security adviser to President Barack Obama; political activist and actress Alyssa Milano; Victoria Reggie Kennedy, the widow of Sen. Edward M. Kennedy (D-Mass.); and Sen. Tammy Duckworth (D-Ill.).

There are still looming questions about Biden — his propensity for gaffes, the moments when he searches for words or, on Monday, his struggle to come up with well-worn phrases in the Declaration of Independence before shouting, “You know . . . the thing!” — but his campaign has found unmistakable new momentum that it is hoping to carry into the elections Tuesday.

Not everyone was playing to his benefit, however. Sen. Elizabeth Warren (D-Mass.) and former New York mayor Mike Bloomberg both pledged Monday to stay in the race.

“I’m in it to win it,” Bloomberg said. He has poured hundreds of millions of dollars into the primary contest and will appear on ballots for the first time Tuesday.

Warren’s current plan is to remain an active candidate all the way to the convention, even if she is not accumulating significant numbers of delegates. She says that will give her some leverage over the party’s platform and any other issues that might be at the forefront in Milwaukee.

She continued her refrain Monday that her operation is built “for the long haul,” and she received an endorsement from Emily’s List, which works to elect women in politics.

The super PAC supporting her candidacy is raising money for a big TV ad buy in states like Arizona, Florida, Illinois and Ohio, which vote March 17, according to a person familiar with Persist PAC’s operations who was not authorized to openly discuss the group’s strategy and spoke on the condition of anonymity.

Sanders remains perhaps the most formidable force in the race, finishing in the top two in all four early states and holding on to a polling advantage in numerous delegate-rich states that will vote Tuesday. He also has a significant financial edge and a grass-roots army that Biden so far has lacked, and he was defiant Monday in the face of the moderate consolidation.

“Look, we are taking on the establishment. And I fully understand — no great surprise to me — that establishment politicians are not going to endorse us,” Sanders told reporters in Salt Lake City, just minutes before the news broke that Klobuchar was ending her campaign and planning to support Biden.

“The establishment will rally around the establishment candidate,” he said. “That’s the simple reality.”

President Trump, who has repeatedly tried to offer sideline commentary about the Democratic contest, tweeted with glee: “They are staging a coup against Bernie!”

What began a year ago as a historically large and diverse Democratic field has quickly narrowed to five remaining contenders. With the two youngest major candidates dropping out, all of the remaining major candidates are septuagenarians.

In Tuesday’s primaries, 1,357 delegates are up for grabs, nearly a third of the total and the most of any single day in the Democratic race. California and Texas are the biggest prizes, but delegate-rich states like Virginia, North Carolina, Massachusetts and Colorado will also vote.

Top advisers to the campaigns agree that Sanders is poised to win a big delegate haul, and the main question is whether it will set him on an insurmountable course toward the nomination.

“After watching this campaign unfold, after watching 10 debates, there’s no way you can rationally believe that Joe Biden is strongest candidate to defeat Donald Trump,” said Ari Rabin-Havt, a deputy campaign manager for Sanders.

Shortly after Reid announced his endorsement of Biden, Faiz Shakir, who is Sanders’s campaign manager but also worked for Reid, wrote on Twitter: “Disappointing. I’ll forever have respect and love for Senator Reid. But I’m old enough to remember when he thought Biden’s ideas were worthy of being put in a fireplace.”

Biden is hoping that the momentum from his romping South Carolina win, and the party’s efforts to consolidate behind him, will be enough to offset his campaign’s continuing deficiencies. He is spending the least on ads, has one of the thinnest organizations and saw his candidacy stall at a time when most other campaigns were banking early votes.

Biden hopes to win among Southern states that more closely mirror South Carolina, where he won nearly 50 percent of the vote Saturday, in part because of his giant lead among African American voters.

His campaign has since received a large fundraising boost, with $5 million coming Saturday and at least $5 million Sunday — all told, more in two days than it raised in all of January.

But the money came too late to make a major advertising splash in the Super Tuesday states, and instead Biden was hoping to be carried by momentum and endorsements.

“The country is hungry — hungry, hungry, hungry — to be united. Most Americans, they don’t want a promise of a revolution, they want a guarantee of results,” Biden said earlier in the day during a campaign stop in Houston. “. . . We need real results, and we need them now. I’ve done that my whole career.”

On CBS on Monday morning, Biden said his surge in support occurred in part because candidates down the ballot are wary of running with Sanders as the Democratic standard-bearer.

“I think there’s an awful lot of people who are running for office who don’t want to run with Bernie at the top of the ticket as a self-proclaimed socialist,” he said.

The party’s donor class also began to rally to Biden’s side.

A handful of key Democratic donors who were raising big checks for Buttigieg or considering publicly endorsing Bloomberg said they were throwing their support to Biden on Monday — and were persuading others to do so, too.

On Monday, a longtime Obama donor in Northern California who had given to multiple candidates during the primaries sent emails urging party donors waffling between Bloomberg and Biden to coalesce around Biden. And on Wall Street, Buttigieg donors were calling one another to say Biden was the best alternative, barring a Bloomberg upset Tuesday.

One longtime Democratic donor in New York, who spoke on the condition of anonymity to describe private conversations, said there were meetings scheduled with major Wall Street financiers starting Sunday through Wednesday, out of anxiety about Buttigieg’s fourth-place finish in South Carolina and concern over his chances on Super Tuesday.

Klobuchar’s early exit means that her home state of Minnesota, which has 75 delegates at stake in its primary, is up for grabs Tuesday. Sanders won the state when it held caucuses in 2016 and had closely trailed Klobuchar in recent polls. He held a rally in Minneapolis on Monday night.

Klobuchar’s decision came after she finished near the bottom in the South Carolina primary Saturday. The senator had briefly surged in some polls after a strong performance at the Feb. 7 Democratic debate in Manchester, N.H. Her campaign had dubbed the boost “Klomentum” and hoped it would catapult her into the top tier of candidates through the rest of the early-nominating states.

Klobuchar finished in third place in the New Hampshire primary, but a disappointing sixth place in the Nevada caucuses and in South Carolina. She had planned an aggressive travel schedule in advance of Super Tuesday but dropped out before she could execute it.

Annie Linskey, Dan Balz, Michael Scherer, Cleve R. Wootson Jr., Michelle Ye Hee Lee, Amy B Wang and Sean Sullivan contributed to this report.