The man who is the very richest undecided voter in Madison Square Garden on Thursday night is slumped in a chair in a swanky private lounge for very special Republicans. He has a tattoo on his neck and huge diamonds in his ears, a Yankees cap on his head. He has his cell phone in one hand and his BlackBerry in the other, tabbing back and forth between the two. The waiters and security guards recognize him. The other guests in the suite, men in perfectly cut suits and women with lacquered hair and pricey shoes, do not.

In a while, Republican National Committee Chairman Ed Gillespie himself is going to come along and put an arm around this man, and escort him onto the convention floor, where the two will stop and chat, creating a mob scene of the curious and the cameras that will have security guards screaming frantically into their microphones. The Texas delegates, in their Lone Star shirts and 10-gallon hats, will stop their Dubya cheers and turn to gape.

But first, we have to ask: What should we call you?

"You can call me Sean," he says, "or you can call me P. Diddy."

His mother named him Sean John Combs, and then he was Puff Daddy, back in the J.Lo days. He lost that name when people started to boo him, after that bit of trouble with the gun in the nightclub. Now Mr. Diddy, multimillionaire mogul, rapper, clothing designer, restaurateur and father of three, has decided to extend his brand of street chic. He has become a political activist, taking $250,000 of his own money to found a new organization called Citizen Change.

He boasts he will drive up voting among the 45 million hip-hop young, who are, he says, "disenfranchised and so, so, so [angry] at being on the sidelines."

He has a shocking slogan -- "Vote or Die" -- plastered on 70,000 T-shirts now selling for $30 at stores from Macy's to J.C. Penney. In the promotional materials, 50 Cent wears his XXL and Mariah Carey wears one that is really tight above her teeny jean shorts.

"I wanted something dramatic, that would strike up controversy," says Diddy. "You can't not vote, is what I'm saying. People have died to have that right. When the president is running your country, he is running you, closing you out of a hospital, or taking you to war. Not just Bush. Every president." The way he sees it, the hip-hoppers already are wearing his Sean John clothes and buying his Bad Boy records and letting him tell them what's cool. They'll listen up when he tells them to register and get to the polls, not that he will be taking them personally, in his tricked-up Lincoln Navigator.

He sees himself as the third rail of American politics. "I'm their worst nightmare," Diddy declares, referring to the two political parties. "I'm closer than Kerry or Bush to these 45 million votes."

Diddy takes great pains to make it clear that although he voted for Gore in 2000, he is a registered independent in New York. A trial on gun charges may not ruin his reputation, but Diddy doesn't want anyone to think he's a Republican or a Democrat.

He went to the Democratic convention in Boston, and now he is here with his latest marketing scheme. "I'm going at it hard," he says. He started early in the day, with a couple of morning show interviews. Then he paired up with Paris Dennard, a 22-year-old black delegate from Arizona, and took him to a Harlem community center where Diddy himself once played on the basketball team, to hear from kids there. An MTV crew trails him, taping the tour for a voting special that will air in October.

At the center, the Minisink Townhouse, Diddy holds the mike. His hair is cut in a Mohawk, because he is in "warrior mode." Jad Joseph, 17, gets right into it with Dennard, demanding to know why Republicans are against gay rights and expanding stem-cell research.

Dennard compliments Joseph on how engaged he is. Diddy stands impassively, recording. If only more commentators would meet fine young men like Joseph, Dennard says, the stereotype against African American men might fall away.

Diddy cuts in. "Let's just answer the question," he commands Dennard. "You come up here to hear about these issues, and I told you, you are not really gonna get away with spinning stuff."

After he referees some more exchanges, Diddy races back downtown to his office, then pops into his Sean John boutique, set to open next week in midtown. Back in the Garden, he checks the path of Hurricane Frances on his BlackBerry, orders the boarding up of his beachfront mansion in Miami. By Thursday night, he is sequestered in Gillespie's suite, scheduled to interview the chairman, California Gov. Arnold Schwarzenegger and Maryland Lt. Gov. Michael Steele for the MTV special and then watch President Bush deliver his speech

He paired up with MTV to embed a voting message throughout the Video Music Awards awards last weekend. Next week, he sends street crews out to paper the hot spots of cities with Citizen Change's Vote or Die fliers, just like the record companies do when they want to promote a new recording.

Because Diddy is a smart mogul, Citizen Change is staffed with talent. Executive Director Alexis McGill used to be the political director for Russell Simmons and an instructor in political science at Wesleyan University. Political adviser Michael Brown is a Washington lobbyist and the son of the late Ron Brown, former commerce secretary and Democratic Party chairman. "And if you use something I say," says Brown, "can you mention possible mayoral candidate in 2006" in Washington?

Because he is Diddy, he moves with a ponderous entourage. There are two bodyguards and two publicists, a personal photographer to shoot an upcoming coffee-table book, "a sort of visual biography," the photographer explains. There is a personal videographer as well, to document his life and mine "valuable footage," an aide explains.

The MTV crew has more than a dozen people, and the posse can move only so quickly. Gillespie wants to escort Diddy around the convention floor, to maximize the Republicans' visibility in trying to reach out to minority voters. The RNC wants to have Gillespie interview Diddy and put it right up there on the big screens inside, where the television broadcasts might get that image into homes all over America. The Diddy people want to make sure the RNC isn't using him. Diddy listens carefully to the back and forth, then tells his team: "I'm not politically savvy enough to figure all this out. You do it."

In the meantime, he watches the broadcast of his interview with a local station and asks for some espresso.

The dispute resolved, Gillespie arrives at the lounge, throws an arm around Diddy and the camera lights switch on. Everybody starts walking backward through the hall. Diddy has the mike. "How you think it's been going?" he asks Gillespie.

"I think it's been great!" the chairman answers.

As they make their way around the convention floor, news photographers rush over. The scrum forms.

A couple of delegates stand on their tiptoes to figure out what's going on.

"Who's the guy in the green tie?" one man asks.

"I think it's the governor of Colorado," a second responds. "Yeah, that's definitely who it is."

Who's the other guy, the first man asks.

No idea, says the second.

"That's P. Diddy," says a woman from Louisiana. "Don't you know P. Diddy?"

"Huh," says the first man. "What's he doing diddlin' around here?"

An MTV crew films P. Diddy with RNC Chairman Ed Gillespie on the convention floor at Madison Square Garden last night. P. Diddy, above, outside Minisink Townhouse in Harlem yesterday, where he was filming with an MTV crew for a voting special. Diddy wields the microphone, below, as Arizona delegate Paris Dennard, right, talks to young potential voters.