DALLAS -- Former Vice President Joe Biden's Democratic presidential bid picked up steam on Monday with the endorsements of two former 2020 rivals - Pete Buttigieg and Amy Klobuchar - who planned to join him on the stage at a rally on the eve of the Super Tuesday primary elections
But Biden still faces a challenge from billionaire former New York Mayor Michael Bloomberg among voters hoping the party will nominate a moderate.
Klobuchar, a senator from Minnesota, will become the third 2020 candidate in as many days to drop out of the race when she announces the suspension of her campaign in Dallas, where she will also publicly back Biden, a Klobuchar aide said.
Buttigieg, a former mayor of South Bend, Indiana, who ended his White House bid on Sunday, also plans to endorse Biden in Dallas, a top adviser said.
Biden is fresh off a resounding victory in Saturday's South Carolina primary and is aiming for a strong showing on Super Tuesday against Senator Bernie Sanders, the national frontrunner and a self-described democratic socialist from Vermont.
The Super Tuesday contests offer the biggest one-day haul of the 1,991 delegates needed to win the party's nomination at its national convention in July, with about 1,357 delegates, or nearly one-third of the total number, up for grabs.
Fourteen states - California, Texas, Virginia, Massachusetts, Tennessee, Alabama, Arkansas, Oklahoma, Minnesota, Vermont, Colorado, Utah, North Carolina and Maine - as well as American Samoa and Democrats living abroad cast ballots on Tuesday. (The primary for expatriate Americans is scheduled to run through March 10.)
Bloomberg, a late entrant to the race, will make his ballot-box debut. He is betting the $500 million of his own money he has poured into his campaign will allow him to make up for not competing in Iowa, New Hampshire, Nevada or South Carolina.
Five candidates - Biden, Bloomberg, Sanders, Senator Elizabeth Warren of Massachusetts and US Representative Tulsi Gabbard of Hawaii - remain in the running for the nomination to take on Republican President Donald Trump in November's election, down from more than 20 earlier in the campaign.
Bloomberg and Biden have emerged as the main contenders for the votes of moderate Democrats, while Sanders is the progressive front-runner nationally, eclipsing Warren.
Biden's high-stakes triumph in South Carolina, where his campaign had said his popularity with black voters would propel him to victory after early disappointing finishes in Iowa and New Hampshire, helped winnow the field.
Billionaire environmental activist Tom Steyer also gave up his campaign on Saturday night after a third-place finish in the Southern state in which he had invested most heavily.
One of Buttigieg's top fundraisers, who spoke on condition of anonymity, said some of his supporters planned to donate to Biden's campaign.
Sanders' momentum might not be easily slowed. On Monday, his campaign downplayed the efforts by moderates to present a united front.
"The establishment is nervous, not because we can't beat Trump, but because we will," said Sanders' campaign manager, Faiz Shakir. "And when we do, the Democratic Party will again be a party of the working class."
It was not immediately clear who would immediately benefit from the departures of Buttigieg and Klobuchar. A Morning Consult poll taken Feb. 23 to 27, for example, before Buttigieg exited the race, showed that 21 percent of his supporters named Sanders as their second choice, 19 percent picked Biden, another 19 percent chose Warren and 17 percent favored Bloomberg.
Biden still lags his rivals in spending and organization in Super Tuesday states and beyond, but his campaign said on Sunday it had raised more than $10 million over the preceding two days.
Endorsements of the former vice president from elected officials and community leaders poured in on Monday as Democrats who believe a moderate is the best candidate to defeat Trump tried to circle the wagons around Biden.
Backing from Ohio Democrats including Representative Marcia Fudge and former Cincinnati Mayor Mark Mallory added to endorsements from Senator Tim Kaine and state House of Delegates Majority Leader Charniele Herring of Virginia. In Colorado, Denver Mayor Michael Hancock has backed Biden. In California, Representative Gil Cisneros is supporting the former vice president.
On Wednesday, Hollywood mogul Sherry Lansing is hosting a fundraiser for Biden featuring Senator Dianne Feinstein of California and Los Angeles Mayor Eric Garcetti.
Supporters of Sanders and Warren also rushed to weigh in for their candidates. The progressive magazine "The Nation" endorsed Sanders, and the women's fundraising organization Emily's List endorsed Warren.
Former President Barack Obama planned to wait until after the primaries to endorse a candidate, a source familiar with his thinking said.