Biden, the former vice president, squares off against Senator Bernie Sanders on Tuesday (March 10) 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.
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.
"The days of Donald Trump's divisiveness are soon going to be over," Biden said.
"And when he's gone, he's gone, we'll get back on the road of building that more perfect union." 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.
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.
Thirty percent said they would vote for Sanders, unchanged from the previous poll.