In endorsing Sam Katz for mayor, John White Jr. tried to prevent his decision from being seen as a personal rejection of John Street.

But the biggest event of the campaign to date could not be seen any other way.

How else do you explain one black Democrat refusing to support another, even though the two men rarely had an unkind word to say about each other through a long and spirited primary? How else do you explain that Democrat opting instead to back a white Republican who ran commercials labeling him a dangerous incompetent?

At the news conference yesterday, White called the Democratic mayoral nominee "a noble person."

Then, in the next breath, he criticized him, saying that he could never get Street, during a summer of off-and-on negotiations, to talk about substantive issues related to governing the city.

And what did Street want to talk about? "When was I going to endorse him," White replied curtly.

Street, it should be noted, hotly disputes this account.

Listening to White, and the people around him, you get a sense of how it got to this point.

During the primary, White watched Street at numerous candidate forums, cataloged his absences at others, and came to have growing doubts about the kind of mayor Street would be.

He wondered whether the former City Council president, whom he considered a friend, had the temperament to reach out beyond his inner circle? Or the commitment to try to do more than balance the books?

Those misgivings grew after the votes were counted in May and White finished a disappointing third. His reservations were reinforced by his associates, a group of about 40 friends, activists and labor leaders, many of whom had their own problems with Street.

"The White campaign [organization], in some ways, was always less a traditional campaign than a ministerium," said Howard Cain, a White adviser. "That congregation stayed together after the primary."

And then along came Katz.

He talked to White on numerous occasions, discussing his approach to governing and explaining why he had felt it necessary to attack him in the spring. He talked to White's father, his supporters, his advisers.

"Sam Katz lobbied every person close to John White," said Obra Kernodle, a White confidant. "For some odd reason, we didn't feel that kind of intensity from John Street."

The contrast between Katz's ardent wooing and what the White people saw as Street's sense of entitlement only served to underline the group's doubts about Street.

But for White himself, a lifelong Democrat who made it clear yesterday that he was backing every Democrat other than Street, the choice remained a tough one. He was, says Kernodle, in agony.

"John White desperately wanted Street to give him a hook to hang his endorsement on," Cain said.

White never found that hook. In the end, said one adviser, he came to see Street as a status quo candidate, someone "who didn't bleed."

When White made his move yesterday, his emotions occasionally seeped through. Asked whether he had notified Street in advance, White replied, "He'll read about it."

The fervent hope in the Katz camp is that White's endorsement will signal progressive whites, who might be reluctant to vote against a black and for a Republican, that this election is less racial and less partisan than usual - and that a vote for Katz is acceptable. The Katz people also hope that it might help keep Street's share of the black vote below 90 percent.

Democratic strategists would like White to be seen as a sore loser motivated by narrow, selfish concerns. The Democratic candidate himself went a step further, accusing White of having made a "backroom deal" with Katz. This, it should be noted, White denies.

When pressed, Street said that he had no knowledge of a deal, only the belief that there had to have been one since "this could not have happened on the basis of the issues." He spoke of his own unwillingness to make deals, then acknowledged that White had never asked him for anything.

No matter. It's a boost for Sam Katz. The only question is how big.

And it's something John Street never should have let happen.