Black voters will decide this election. The math is simple, and I’ll try to keep it that way.

The Hispanic vote split four years ago, canceling itself out. Mayor Bloomberg won the white vote (59%) and Mark Green won the black vote (71%). Because white voters far outnumber other voting blocks, Mr. Bloomberg won.

Fernando Ferrer’s quest to become the city’s first Latino mayor guarantees Hispanic voters won’t split this year. Mr. Bloomberg is counting on a division among black voters instead. So far he’s in good shape.

Recent polls suggest the candidates are splitting the black vote almost evenly. That’s fine with Mr. Bloomberg. He just needs to keep black voters where they are – divided.

Mr. Ferrer has a tougher task. He needs a big win among black voters – his supporters acknowledge at least 60% is required, but 70% might be a more accurate number. With Mr. Bloomberg likely to trounce Mr. Ferrer among white voters, only a coalition of black and Hispanic support can put Mr. Ferrer over the top.

While their influence has been declining, white voters still make up about half the turnout. Two recent polls show Mr. Bloomberg above 60% among white voters.

Hispanic voters will probably make up a quarter of the turnout because of Mr. Ferrer’s presence in the race, up from 18% of the 2001 election. Polls show that Mr. Ferrer enjoys about 60% support among Latinos, and that will likely climb higher – no matter how much time Mr. Bloomberg spends practicing Spanish in the backseat of his chauffeured Chevy Tahoe.

Both candidates have been careful to court many ethnic groups, including the city’s growing Asian population. But at 25% of the likely turnout, black voters are the most influential target.

Mr. Ferrer’s hopes black voters will follow tradition by overwhelmingly supporting the Democrat. Take the last four elections. Black voters went for the Democrat by more than 70% in 2001 and 1997, and more than 90% when David Dinkins was the candidate in 1993 and 1989.

Mr. Ferrer’s supporters understand this math very well. They learned how instrumental black voters could be when the Reverend Al Sharpton’s last-minute endorsement brought Mr. Ferrer across the 40% Democratic primary threshold that avoided a runoff last month.

But the primary was easy compared to what Mr. Ferrer is up against now – a $100 million campaign that is poised to deny Democrats their dream of a black-Latino coalition. Mr. Bloomberg entered office with automatic appeal to black New Yorkers for one simple reason: He’s a Republican mayor who isn’t Rudy Giuliani. The new mayor immediately built on that incidental goodwill by cultivating black leaders who’d been consistently ignored by the man now known as America’s Mayor.

Even the ill-conceived plan for a West Side stadium appealed to many black politicians, because construction unions promised to welcome black workers who have historically been shut out of big projects – and the lucrative jobs that accompany them.

To his credit, Mr. Bloomberg’s police department has avoided any major racial skirmishes. And earlier this year – when racial incident threatened to erupt in Howard Beach – Mr. Bloomberg raced to the scene and neutralized Rev. Sharpton with a single strategic phone call. Hizzoner was praised simply for caring more than Mr. Giuliani ever seemed to care.

But the mayor’s cadre of campaign consultants doesn’t always handle the black community with proper finesse. Mr. Bloomberg’s flat-footed decision to skip the Apollo Theater debate – without acknowledging the inherent slight to Harlem residents – stands out as his campaign’s first palpable error.

Mr. Ferrer quickly pounced on what Rep. Charles Rangel called the “best news” to happen to the Ferrer campaign. But Mr. Bloomberg is counting on his own black supporters to mute the damage.

Just hours after Rev. Sharpton recorded an attack ad blasting the mayor for the debate snub, the Bloomberg campaign unveiled a radio ad that features Harlem minister Calvin Butts praising the mayor’s commitment to the nation’s black capital.

The symbolism of black leaders divided isn’t enough for Mr. Bloomberg. He needs black voters to stay divided.

This week’s dueling radio ads portend the beginning of an all-out fight for black voters that will dominate the next five weeks. From public schools to housing, the candidates will focus on issues that matter most to this year’s crucial swing voters.

And President Bush’s low standing among black New Yorkers is among the reasons that Mr. Ferrer keeps linking Mr. Bloomberg to the White House.

Independence Party activist Lenora Fulani generally deserves disdain rather than credit, but she made an accurate observation while courting black voters for Mr. Bloomberg in 2001: “If we give him 20%, then we win because it makes the black vote an unpredictable vote in the future. The black community doesn’t realize how valuable unpredictable votes are. People have to negotiate with you if they don’t know what you are going to do.”

Mr. Bloomberg went on to win 25% of the black vote. The future is now. And the black vote is unpredictable.

Mr. Goldin is a host of NY1’s “Road to City Hall,” which airs weeknights at 7 p.m. and 10:30 p.m.