Amy Klobuchar, Pete Buttigieg rally behind Joe Biden on eve of crucial Super Tuesday contests

Show Caption Hide Caption What is Super Tuesday and why is it important? A previous version of this video displayed an incorrect date in a graphic. It was 1988 when 14 Southern and border states held their primaries.

Democratic presidential candidate Joe Biden got a boost ahead of the Super Tuesday nomination contests when Sen. Amy Klobuchar and Pete Buttigieg gave him their endorsements Monday after ending their own White House bids.

Fresh off a huge win in South Carolina, Biden seeks to position himself as the most viable alternative to Sen. Bernie Sanders, the Democratic front-runner who is running on a progressive agenda that includes a "Medicare for All" proposal and a plan to erase student debt.

Buttigieg and Klobuchar each flew to Dallas on Monday. Biden was scheduled to hold a rally there on the eve of Super Tuesday. At stake in Texas are 228 delegates – the third-most delegates available in the Democratic nominating contest. On Tuesday, 14 states will determine one-third of the pledged delegates – 1,344 out of the 1,991 needed to win the nomination to challenge President Donald Trump in November.

Sanders has led polling averages in the biggest prize of California: 415 delegates are up for grabs. The Vermont senator, who held the most delegates heading into Tuesday, tweeted Monday that he doesn't believe "we will defeat Donald Trump with a candidate like Joe Biden who supported the Iraq War."

But many moderate Democrats and establishment party figures feared that if Sanders, a Democratic socialist, won the nomination, he wouldn't be able to attract the broad coalition the party needs to beat Trump. Biden has criticized Sanders' plans such as "Medicare for All" and erasing student debt as too costly.

Another wild card is that former New York Mayor Michael Bloomberg will begin appearing on ballots Tuesday after spending $539 million through Thursday to blanket the country with advertising.

To combat Sanders' grassroots campaign and Bloomberg's advertising, Biden spent Monday collecting high-profile endorsements, including former Senate Majority Leader Harry Reid of Nevada. Reid called Biden the "best able" to defeat Trump.

Austin Mayor Steve Adler, who earlier endorsed Buttigieg, threw his support Monday behind Biden, saying "it's time for the party to consolidate."

The twin departures of Klobuchar and Buttigieg in less than 24 hours offered Biden an opportunity to unite moderate voters behind his candidacy.

Klobuchar pitched herself as a moderate with Midwestern appeal and the political "grit." She enjoyed a boomlet after the New Hampshire primary, where a surprise third-place finish gave her a needed fundraising boost and fresh political momentum. Klobuchar said the New Hampshire results showed she could beat expectations and build a broad coalition, as she had in her Senate races.

"I defied expectations, and I won. And I have done it over and over again in the reddest of red districts and the bluest of blue districts,” she told supporters in New Hampshire.

But her "Klomentum" seemed to evaporate as quickly as it emerged. She garnered only 3% of the vote in Saturday's South Carolina primary, a result that all but closed off her path to the nomination.

Buttigieg, a former Navy intelligence officer who served in Afghanistan, was one of the few military veterans in the race. He also offered academic credentials as a Rhodes Scholar with degrees from Harvard University and the University of Oxford who speaks multiple languages. He inspired unity and inclusiveness as an openly gay candidate.

But after winning the Iowa caucuses, he found it difficult to gain traction with younger voters and placed a disappointing fourth Saturday in South Carolina. In his departure speech Sunday in South Bend, Buttigieg said he would "do everything in my power to ensure that we have a new Democratic president come January.”

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Klobuchar invested most of her time and money in Iowa, where she finished in fifth place, and New Hampshire, where she snagged nearly 20% of the vote.

But just a few days after the New Hampshire primary, Klobuchar stumbled during an interview with Telemundo when asked if she could name the president of Mexico. "No," she responded in a lapse that came back to haunt her during the pivotal Feb. 18 Democratic debate.

And the New Hampshire bounce was not enough to help her campaign scale up as the race moved to Nevada and South Carolina. She finished sixth in the Nevada caucuses.

Klobuchar's rivals, including Biden and Sanders, were far better organized in those two states. And they enjoyed broader support among Latino and African-American voters, key constituencies she was unable to win over.

Klobuchar, 59, has been the senior U.S. senator from Minnesota since 2007 and is the state’s first female senator. She launched her campaign in the middle of a snowstorm on the Mississippi River shore.

"I don't come from money," Klobuchar said as snow piled on the crowd. "But what I do have is this: I have grit. I have family. I have friends. I have neighbors. I have all of you who are willing to come out in the middle of the winter, all of you who took the time to watch us today from home, all of you who are willing to stand up and say people matter."

Klobuchar cast herself as the best alternative to liberals in the race, Sanders and Sen. Elizabeth Warren, touting her Midwestern roots and her ability to work across the aisle to get things done in the Senate.

"I'm someone that can win and beat Donald Trump," she said during the first primary debate last year. "I have won every place, every race, and every time. I have won in the reddest of districts, ones that Donald Trump won by over 20 points. I can win in states like Wisconsin and Iowa and in Michigan."

On some progressive policies, such as “Medicare for All” and access to free college, Klobuchar differed from her more progressive counterparts.

Klobuchar: Trump trying to undermine constitution Democratic presidential candidate Senator Amy Klobuchar says President Trump is trying to undermine the constitution. Klobuchar, in Des Moines, blasted Trump's border emergency declaration, saying "I thought an emergency was floods in Iowa." (Feb. 22)

On the campaign trail, Klobuchar at times had to contend with reporting that dubbed her one of the “worst bosses” in Congress, a designation Politico gave her in 2018.

Opinion: Klobuchar's bad-boss flaws are disturbing. Should Democrats write her off?

The New York Times in 2019 reported multiple cases of her “unethical” conduct as an employer in February. Among the claims: Klobuchar was known to “throw objects in frustration, including binders and phones, in the direction of aides.” In one notorious anecdote, Klobuchar reportedly ate her salad with a comb when an aide failed to bring her a fork.

“Yes, I can be tough, and yes, I can push people,” Klobuchar told reporters after initial allegations circulated. “I have high expectations for myself. I have high expectations for the people that work for me. But I have high expectations for this country.”

Contributing: Rebecca Morin