Democratic U.S. presidential candidate and former U.S. Vice President Joe Biden speaks during a campaign event at Booker T…
Democratic U.S. presidential candidate and former U.S. Vice President Joe Biden speaks during a campaign event at Booker T. Washington High School in Norfolk, Virginia, U.S., March 1, 2020.

WASHINGTON - Former U.S. Vice President Joe Biden picked up three key endorsements Monday from former rivals for the Democratic presidential ticket, adding momentum to his campaign ahead of  voting in primaries in more than a dozen states known as Super Tuesday.

U.S. Sen. Amy Klobuchar ended her bid for the Democratic presidential nomination Monday and, along with former Texas Congressman Beto O'Rourke, endorsed Biden at a rally in Dallas. Klobuchar's withdrawal came a day after another moderate Democrat, former South Bend, Indiana, Mayor Pete Buttigieg, ended his campaign. He also endorsed Biden on Monday.

Klobuchar, from the Midwestern state of Minnesota, positioned herself as a practical pragmatist in the Democratic presidential nomination race, adopting decidedly moderate positions on several issues, including opposition to a government takeover of health care insurance in the United States.

But despite a brief surge in political surveys, she did no better than third-to-sixth-place finishes in the first four state nominating contests last month. Pre-election surveys for Super Tuesday voting showed her ahead only in her home state.

Biden swept to his first primary election victory in three runs for the U.S. presidency on Saturday, a convincing win in South Carolina.

In addition to Klobuchar and Buttigieg, billionaire environmentalist Tom Steyer also dropped out of the race after Biden's strong showing in South Carolina.

Democratic presidential candidate Sen. Amy Klobuchar, D-Minn., greets supporters during a rally Saturday, Feb. 29, 2020, in Richmond, Va.

The trio of withdrawals leaves four major Democratic contenders: current leader Sen. Bernie Sanders of Vermont, a self-declared democratic socialist; Sen. Elizabeth Warren of Massachusetts, another liberal who has fared poorly in early voting;  former New York Mayor Michael Bloomberg, a billionaire whose name is appearing on ballots for the first time Tuesday; and Biden, who served two terms as the country's vice president under former President Barack Obama.

Polling shows Biden winning seven of the 14 states voting Tuesday, with Sanders coming out on top in six states. Minnesota is now uncertain with Klobuchar's withdrawal. But Sanders is far ahead in California, the most populous state in the U.S. and the biggest prize in Tuesday's voting.

A total of 415 delegates to July's National Democratic Convention are at stake in California, which could give Sanders a substantial edge in the race for delegates no matter how many other states Biden might win in the Tuesday balloting. Candidates are awarded pledged convention delegates based on their vote counts in the state-by-state elections, but get none if they do not win at least 15% of the vote.

But Biden is already pressing his case that Sanders at the top of the Democratic ticket would prove a disaster in some parts of the country for other Democratic candidates in the November election.

“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,” Biden said in an interview with CBS News.

"The establishment is getting very, very nervous," Sanders told supporters at a Monday rally in Utah.