Biden builds on moderate coalition, collecting endorsements from former 2020 candidates

Following a commanding string of victories on Super Tuesday, former Vice President Joe Biden is continuing to build a coalition within the moderate wing of the Democratic Party, as he gears up for what is now essentially a two-person race between himself and liberal stalwart, Vermont Sen. Bernie Sanders.

New Jersey Sen. Cory Booker on Monday added his endorsement to the chorus of former presidential candidates backing Biden-- the latest prominent black lawmaker to do so. It is also part of a series of endorsements on the eve of primary voting in Michigan--a state with a large black electorate.

"Joe is building the kind of campaign we need to win -- a campaign based on shared values, built by the kinds of multi-racial, multi-ethnic coalitions that have always been at the backbone of every movement for progress in our history," Booker wrote in a letter to supporters on Monday. "Joe is building the kind of campaign that will do more than remove one guy from one office. He will lead the Democratic Party to victory in races up and down the ballot across the country this November.

During the tense Democratic contest, Booker--a rising star within the party-- and Biden heatedly debated the veteran lawmaker's support of the 1994 crime bill when he was a senator, which critics say helped lay the groundwork for mass incarceration. Biden has since apologized for some of his criminal justice approaches and released a criminal justice plan that, in some ways, would reverse measures he helped put in place.

California Sen. Kamala Harris, whose tenure in the 2020 race included a series of tense back-and-forth exchanges with Biden over his opposition to busing during desegregation, endorsed the former vice president on Sunday morning.

“When I started my run for president, I said America needs a president who reflects the decency and dignity of the American people; a president who speaks the truth; and a president who fights for those whose voices are too often overlooked or ignored. I still believe that to this day. That is why I am proud to announce I am endorsing my friend, Vice President Joe Biden, for President of the United States," Harris wrote in a statement released Sunday morning.

Harris also recorded a video endorsing Biden. She and Booker will both be in Detroit on Monday to campaign alongside Biden.

"I believe that he is a man who has lived his life with great dignity. He is a public servant who has always worked for the best of who we are as a nation and we need that right now. There is so much at stake in this election, guys. So join me in supporting Joe and let’s get this done," Harris says in the video.

Former Massachusetts Governor Deval Patrick, who entered the 2020 contest in November, far later than the rest of the field and who also has deep ties to former President Barack Obama, endorsed Biden on Friday. Patrick will head to Mississippi this weekend to campaign alongside Biden, ahead of the state's primary on Tuesday -- where Biden is again expected to show his strength among African-American voters. In 2016, just over 70% of Mississippi's electorate was African-American, according to ABC News exit polls.

"This is a moment of profound consequence in America. At a time when our democracy is at risk, our economy is not working for many Americans, and our role in the world is unsteady, America needs a unifying and experienced leader, who can and wants to make life better for everyone everywhere," Patrick wrote in a statement released Friday. "Joe Biden is that leader. I am today proud to endorse him for the Democratic nomination for President."

While his bid was short-lived, Patrick was seen as a fast-rising star in the Democratic Party that could potentially challenge Biden's support within the African-American community. However, he was unable to gain traction in a crowded field and his late entry prevented him from amassing the resources necessary to break through in a congested media landscape.

Patrick's endorsement comes just days after Biden emerged victorious in at least 10 states on Super Tuesday, re-shaping the Democratic primary contest and pushing two of his fellow rivals, Massachusetts Sen. Elizabeth Warren and former New York City Mayor Michael Bloomberg, to end their candidacies after disappointing showings.

Bloomberg endorsed Biden the same day he suspended his campaign, while Warren said she's taking time to assess whether or not she will throw her weight behind any of the remaining candidates.

"I want to take a little time to think a little more. I've been spending a lot of time right now on the question of suspending and making sure this works the best we can for our staff, our team and our volunteers," Warren said at a press conference on Thursday in Cambridge, Massachusetts, where she announced her decision to suspend her campaign.

Just prior to Super Tuesday's critical slate of primary contests, Biden got a critical and historic boost from two of the races most prominent moderates: Minnesota Sen. Amy Klobuchar and former South Bend, Indiana Mayor Pete Buttigieg.

Within a span of 48 hours, both candidates suspended their campaigns and flew to Dallas, Texas on Monday to publicly announce their support for Biden, a show of force that further solidified his position as the de facto moderate alternative to Sanders.

"I’m looking for a president, who will draw out what is best in each of us," Buttigieg said standing next to Biden at a barbecue restaurant in Dallas. "And I’m encouraging everybody who was part of my campaign to join me, because we have found that leader in vice president, soon to be president, Joe Biden."

Klobuchar weighed in as well, standing alongside Biden at a rally later Monday evening and further aiding in the coalescing of moderates around the former his candidacy.

"If you are tired of the extremes, you have a home with me, and I think you know you have a home with Joe Biden," Klobuchar said.

Following their lead, former Texas Congressman Beto O'Rourke -- another former 2020 rival -- made a surprise appearance at the Dallas rally, telling the crowd he too was endorsing Biden just hours before the state of Texas began its primary.

Sanders, now looking for a path to re-take the delegate lead from Biden, has sought to paint the endorsements as the Democratic "establishment," uniting behind the former vice president to blunt his momentum, a charge Biden pushed back against on Wednesday.

"The establishment are all those hard working middle class people, those African Americans, the single women in suburbia," Biden told reporters on Wednesday in Los Angeles, after delivering a press statement urging party unity. "They’re the establishment."

ABC News' Zohreen Shah and Molly Nagle contributed to this report.