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 24 hours 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.
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 the Monday night one week ago, 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, the race’s leading progressive, scrambles to make up ground among black voters in Michigan, where they make up a sizable part of the electorate.
The Biden campaign has made a strong showing in Michigan central to maintaining momentum after Super Tuesday, when Mr. Biden captured 10 of 14 states. 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 blocks 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. Booker, despite dropping out before the nation’s first primary contest, had previously campaigned in Detroit this cycle in an effort to pitch himself as the best candidate to re-energize the fractured Obama coalition and restore the black vote in swing states that Hillary Clinton lost in 2016.
Though Mr. Biden has indicated that he would most likely select a woman as a 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 transitioned some of his campaign apparatus into his Senate re-election bid. Though his favorability in the state took a small dip as he campaigned across the country for president, Mr. Booker is still immensely popular in his home state at a time when New Jersey is shifting ever bluer, becoming one of the most Democratic states in the country. Of the 12 members the state sends to Congress, only two are Republican.
New Jersey is one of the last states to vote in the Democratic primary, and its 126 delegates who are up for grabs 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, and with pledged super delegates possibly tipping one candidate over the edge, it’s unlikely that the state will play a major role in helping to decide the nominee.