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Booker announced his endorsement at an event in Harlem.

"What I'm here to emphasize today is Mayor Bloomberg is a national leader in the country, making the kind of change that will help us all," Booker said.

Booker is popular across the river, but Bloomberg's main rival, City Comptroller Bill Thompson, says Booker's endorsement does not carry much weight in the city.

"I respect Mayor Booker, but hard-working people who actually live in New York know that we need change and that Michael Bloomberg offers four more years of the same," said Thompson in a statement he released Friday.

State Senator Bill Perkins, a likely Thompson supporter, says he agrees with the comptroller's assessment.

"Just racial, perhaps, on the part of the mayor, that he would go outside of the city to get some support," Perkins said. "But, as far as I'm concerned, I don't believe that that support will measure up to anything meaningful."

"Bill Perkins has a right to his views, but his views are a view of one," countered Bloomberg. "In the end, on November 3rd, we will do a poll as to whether Cory Booker's endorsement means something. I think you will find it means an enormous amount."

Bloomberg's campaign has been aggressively courting black voters, both in Harlem and across the city, chipping away at Thompson's core constituency.

"We are doing a grassroots campaign of the African-American community. Leaving no stone unturned," said Basil Smikle of the Bloomberg Campaign. "The African-American vote is really important to us."

Bloomberg has been praised for healing racial wounds in the city, and in 2005 he won 47 percent of the black vote.

Winning their support this time could be the key to winning the election. Every African-American vote for him means Thompson has to look elsewhere for support.