Senator Cory Booker of New Jersey endorsed former Vice President Joseph R. Biden Jr. for president on Monday, adding to what has become a nearly complete consolidation of support from Mr. Biden’s former top rivals to push him to the Democratic nomination.
Mr. Booker’s endorsement comes one day after Senator Kamala Harris of California endorsed Mr. Biden, and the two senators will appear with him at a rally in Detroit on Monday night. Mr. Booker will also campaign alongside Mr. Biden in Flint, Mich., earlier in the day and attend a fund-raiser with him.
“It a time for us to beat Donald Trump and it became very clear to me that Joe Biden is the right person to do that,” Mr. Booker said in an interview on “CBS This Morning,” after announcing his endorsement in a tweet.
“We have to unify and show our strength and I think this Tuesday could be a pivotal day in our primary progress, but it’s about time that we start unifying as a party and begin the work to beat Donald Trump, and frankly save our nation.”
The event in Michigan, which holds its delegate-heavy primary on Tuesday, will be yet another public show of moderate Democratic support for the former vice president on the eve of a major vote in the presidential race.
It recalls last Monday night, when Senator Amy Klobuchar of Minnesota, former Mayor Pete Buttigieg of South Bend, Ind., and former Representative Beto O’Rourke of Texas all spoke out for Mr. Biden on the eve of Super Tuesday.
Mr. Booker and Ms. Harris were two of the most prominent black candidates to run for president, and their endorsements come as Senator Bernie Sanders of Vermont scrambles to make up ground among black voters in Michigan, where they make up a sizable part of the electorate.
“African-American voters in the South, African-American voters here in Detroit, they have played a pivotal role in my entire lifetime in choosing the Democratic nominee,” Mr. Booker said. He pointed to Mr. Biden’s deep connections with black voters and what Mr. Booker saw as “understanding the issues of race and frankly racial reconciliation and racial justice, and even saying things now about choosing a black woman on the Supreme Court.”
Though six states vote on Tuesday, Michigan, where Mr. Sanders’s surprise victory in 2016 signaled the strength of his insurgent presidential campaign, has become a critical battleground for Mr. Sanders and Mr. Biden to show both their ability to turn out key blocs of support and to expand the electorate in a state that will undoubtedly be a major target for President Trump in the general election.
Mr. Sanders canceled events this weekend in Mississippi to focus his efforts on Michigan, effectively ceding another state in the South to Mr. Biden while redoubling his effort in Michigan to prevent the former vice president from amassing an insurmountable delegate lead early in the primary calendar.
Mr. Biden also picked up two major endorsements, as two national gun control advocacy groups — Everytown for Gun Safety and the Brady Campaign — announced their support for Mr. Biden, evidence that Mr. Sanders’s past positions on gun control could undermine his support on an issue popular with many Democratic voters.
Mr. Booker, despite dropping out of the presidential race before the nation’s first primary contest, had campaigned in Detroit this cycle in an effort to pitch himself as the best candidate to re-energize the fractured Obama coalition and excite black voters in swing states that Hillary Clinton lost in 2016.
Though Mr. Biden has indicated that he would most likely select a woman as his running mate, Mr. Booker’s support for Mr. Biden as the race effectively narrows to two candidates will inevitably draw some speculation as to the New Jersey senator’s future.
Elected to the Senate in 2013, Mr. Booker, the telegenic former mayor of Newark, was long seen as a rising star in the Democratic Party, destined for an eventual national campaign. But he struggled to break through in the crowded 2020 field and ended his campaign early, dropping out of the presidential race three weeks before the Iowa caucuses.
Mr. Biden, who achieved sweeping victories in both the South Carolina primary and in the Super Tuesday contests on the strength of black voters across the country, now counts the backing of all the major former black candidates for president, having earned the endorsements of Ms. Harris and former Gov. Deval Patrick of Massachusetts in recent days.
Mr. Booker is up for re-election in New Jersey this year, and when he dropped out of the presidential race he immediately moved some of his campaign apparatus over to his Senate re-election bid.
Though his favorability rating in the state took a small dip as he campaigned across the country for president, Mr. Booker is still immensely popular at home at a time when New Jersey is shifting ever bluer, becoming one of the most Democratic states in the country. Of the 12 members of the state’s Congressional delegation, only two are Republican.
New Jersey is one of the last states to vote in the Democratic primary, and the 126 pledged delegates up for grabs there in June were seen as a possible final stand for whoever was left fighting for the Democratic nomination. But with the field essentially winnowed to a two-person race, it’s unlikely that the state will play a major role in helping to decide the nominee.
Though Mr. Booker is in the middle of his own campaign, he temporarily turned over his fund-raising list and apparatus to the Biden effort early Monday, sending an email that directed people to the Booker for Senate page with a disclosure: “100% of your contribution will go to the Biden for President campaign.”
The email contained a slightly retooled applause line from Mr. Booker’s own stump speech.
“Because Joe Biden understands that beating Donald Trump is the floor,” Mr. Booker wrote. “It is not the ceiling.”