DETROIT (Reuters) - Seeking a decisive win in the Michigan primary, Joe Biden was joined onstage in Detroit on Monday by two former Democratic presidential rivals, Cory Booker and Kamala Harris, in the latest sign the party establishment is lining up behind him.

Biden, the former vice president, squares off against Senator Bernie Sanders on Tuesday in Michigan and five other states in the battle for the Democratic presidential nomination.

A win by Biden in industrial Michigan, which is the biggest prize in Tuesday’s voting, could leave Sanders and his campaign reeling.

The highly coveted support from Booker and Harris, two black U.S. senators, could give Biden even more currency with African-American voters in Michigan and elsewhere as he seeks his party’s nomination to take on Republican President Donald Trump in November.

“There is one man now who is ready to go to the center of the arena, who is our best shot to beat Donald Trump,” Booker told the raucous crowd of 2,000 at a Detroit high school.

“I believe in Joe. I believe in Joe,” Harris said. “I know Joe. And that’s why I’m supporting him.”

As a marching band played, Biden stood arm in arm with Harris, Booker and Michigan Governor Gretchen Whitmer in a gesture of growing Democratic unity. “Folks, look, we’re going to unite Democrats, Republicans, independents of every stripe. That’s what the nation needs. We’ve got to bring the country together,” Biden said.

Over the past nine days, Biden has powered to the lead in the contest for the Democratic nomination. A wave of endorsements by his former rivals has made him the clear standard-bearer for the Democratic Party’s moderate wing, while Sanders represents the party’s more liberal elements.

The scene in Detroit on Monday night was reminiscent of the day before Super Tuesday, when he was endorsed in rapid-fire fashion by former presidential rivals Pete Buttigieg, Amy Klobuchar and Beto O’Rourke.

Booker and Harris, along with Klobuchar, a senator from Minnesota, are considered potential candidates to be Biden’s running mate on the Democratic ticket should he secure the nomination.

“They’re all capable of being president,” Biden told NBC News in an interview on Monday, adding that it would be “presumptuous” for him to settle on a choice before he is the nominee.

A Reuters/Ipsos national poll released on Monday showed Biden was amassing widening support from Democratic voters.

The poll conducted from Friday to Monday showed that 47% of registered Democrats and independents said they would vote for Biden if their state’s nominating contests were held today, up 7 points from last week.

Slideshow ( 8 images )

Thirty percent said they would vote for Sanders, unchanged from the previous poll.

Biden was apparently attracting supporters of Senator Elizabeth Warren, who bowed out of the race last week but has not endorsed Biden or Sanders.

‘LET ‘EM GO’

Slideshow ( 8 images )

Sanders held a rally on Monday in St. Louis. Missouri, Mississippi, Washington state, Idaho and North Dakota also hold nominating contests on Tuesday.

Sanders returned later to Michigan, where he joined a roundtable in Detroit on the coronavirus. Michigan officials said on Friday they would take steps including having clerks repeatedly sanitize voting booths to alleviate the risks of infection spreading at polling places.

Sanders criticized Trump for suggesting people could still go to work if they contracted the virus, saying it “might be the stupidest advice ever made by a president of the United States.”

Protesters who appeared to support Sanders interrupted Biden’s Detroit speech, unfurling a banner in protest of Biden’s support of a new U.S. trade deal with Mexico and Canada.

“The Bernie bros are here,” Biden told the crowd. “Let ‘em go.”

One of the most talked-about moments of Harris’ campaign was her blistering June debate attack on Biden, when she called his remarks about working with segregationist senators hurtful and questioned his 1970s opposition to school busing.

Booker, too, had been deeply critical of aspects of Biden’s record, including his support of a 1990s crime bill and his stance of marijuana legalization.

Still, black voters have backed Biden overwhelmingly, and propelled him to big victories in South Carolina on Feb. 29 and on Super Tuesday last week. They will be crucial in Michigan, where they comprise almost 14% of the population.

Sanders, who drew huge crowds to weekend campaign events across Michigan, scored an upset win in the state four years ago in the Democratic primary against eventual nominee Hillary Clinton. Clinton would later lose the state by a narrow margin to Trump.

But Sanders still lags well behind Biden among black voters.

On Sunday, he sought to close that gap with an endorsement from civil rights activist Jesse Jackson, who said the senator’s progressive agenda offered black Americans the best chance to catch up socially and economically.