For the want of a birthday party, the empire was lost.

That would make a curious political epitaph for anyone, especially for urban America's most urbane and resilient political boss, Boston Mayor Kevin H. White. And yet at the moment, it has an ominously fateful ring to it.

White, the moody, enigmatic, street-savvy Irish pol who has experienced highs and lows in 15 years as Boston's mayor, is in the fix of his career.

Federal investigators are combing City Hall corridors in the most far-reaching local corruption probe in decades. Newspaper reports say a state ethics panel has found "evidence of possible cash laundering" in connection with a 1981 party planned for White's wife. A bevy of "young Kevin White clones" is hungering for a crack at his job this fall. And the local press, for so long so friendly, has drawn the long knives.

All of this over a birthday bash that never took place?

Not all, perhaps, but the aborted party has become heavy baggage because it symbolizes what critics have been saying for years about White's mayoralty: that midway through his tenure he shucked off his progressive, policy-oriented, open-government style in favor of an imperious, appearances-be-damned "King Kevin" posture as boss of a political machine he built and has oiled with City Hall largess.

"That party . . . is a classic example of what happens when someone holds too much power for too long a time in too small a town," said Sheriff Dennis Kearney, one of the half-dozen candidates planning to run for mayor this year in a field that may or may not include White.

The party was to be held in March, 1981, for Kathryn White's 47th birthday. Invitations went to 1,500 patronage employes, political contributors, City Hall contractors and personal friends. Contributions, it was made clear, were welcome.

In the absence of any other explanation, the assumption of many Boston pols and political observers was then, and remains today, that whatever money was collected above the cost of the event would be used to enrich the White family.

Money flowed in: $122,000, about 60 percent of which came from City Hall employes or their relatives, the state ethics commission reported. Twenty percent came from contractors doing business with the city.

Many of the contributors had never met the object of their generosity, the mayor's wife.

When some disgruntled City Hall employes threatened to picket the event, the press learned of the story, and the mayor--who has said all along that he was not personally involved in planning the affair--abruptly canceled it. All of the money eventually was returned.

But the reverberations had just begun.

The fledgling ethics commission decided to look into the affair. Earlier this year, it slapped White on the wrist in a letter saying he had violated the state's conflict-of-interest code by giving the appearance that he could be improperly influenced by allowing the money to be solicited in the first place.

White accepted the finding, apparently ending the story. But it was not finished. The most intriguing part of the commission's finding was not released publicly at the time. It was turned over to the U.S. attorney's office and recently has been leaked to the local press.

A report in The Boston Globe said the commission found that at least 64 contributors to the party had made cash deposits into their checking accounts about the same time they made out checks in the same amount to the birthday celebration committee.

Where did the cash come from? The contributors--some of whom were $14,000-a-year City Hall secretaries who had given $1,000 to the party fund--reportedly told investigators they had won it at the racetrack or just had it lying around.

The U.S. attorney's office and agents from the FBI, Internal Revenue Service, Postal Service and the Department of Housing and Urban Development, reportedly are investigating whether the money came from a pool of cash from contractors doing business with the city's federally funded housing program.

The feds are investigating the housing program and the city's retirement disability system, which has been the target of allegations that generous lifetime disability pensions have been awarded on the basis of political loyalty to White.

In addition, four top political allies of White and a handful of lesser lights were indicted earlier in the year on extortion and other corruption charges not connected with the birthday party.

White and his lieutenants have called the cash-laundering allegations outrageous and say that the probe is tainted by politics. The U.S. attorney is a Republican, William F. Weld, who they say has ambitions about a U.S. Senate seat.

The White camp has issued subpoenas to track down leaks and has filed complaints with the Justice Department about Weld's investigative techniques. White also has accused The Globe of being out to get him. Last summer, the paper wrote an anybody-but-White editorial, although the 1983 mayor's race then was more than a year away. He accuses the newspaper of "gross exaggerations" and distortions.

In a city that gobbles up political intrigue like so many clams on the half-shell, some think White is trying to stir up the most ancient resentment in Boston politics--enmity between Irish and Brahmin. It happens that Yankee blood courses through the veins of Weld and The Globe, while White has his bona fides as son and grandson of Irish street pols.

If that is his motive, few suspect it will work. "The one thing about Kevin is that he's accumulated too much power to come off as a victim," said Thomas Kiley, a local pollster whose firm has worked for White.

Kiley is among those who believe that White will not make the race. "My sense is that he's too shrewd a politician to even try," he said. "Bostonians are unusually cynical about their local government, but even they have a limit. There's a sense out there that Kevin has gone beyond the pale, that he's made City Hall too much the tool of one man."

Still, others believe just as firmly that the probe is the stimulus White needs to rouse himself from his fitful boredom with the job and vindicate his name.

If he does run, he is not to be counted out, despite negative poll ratings ranging between 60 percent and 70 percent. "My political secret has always been that my opponents outfumble me," he has said.

While the probable field for this fall's nonpartisan primary includes several politicians who could make citywide appeals, it also includes a few with strong neighborhood bases and little room to grow. If White winds up in a runoff with one of the narrow-based opponents, his chances for a fifth term would be enhanced. The nonpartisan primary is in September, with a runoff in November.

In 15 years on the job, White's crowning achievement has been presiding over the renaissance of Boston's downtown and waterfront. Politically, he has been able to turn his imperious style into a plus, offering himself to the voters as someone tough enough to know "which arms to twist and which hands to hold," as one of his campaign lines notes.

But some believe White's style has become counterproductive. The city has just emerged from an 18-month bout with fiscal insolvency, and White laid off 3,000 city employes during the worst of it, including more than 600 police officers and firefighters. Aside from the Roman Catholic Church, few cows are as sacred here as police and firefighters, although municipal watchdog groups long have said that the uniformed departments are overmanned.

White claimed at the time that he had to show the state legislature "blood" if he were to get the rescue bill he wanted. Lots of neighborhood folks claimed he was playing politics with their safety, and demonstrators bearing "Honk If You Hate White" placards poured into the streets.

That crisis has passed. The rescue bill was approved, although not in the form White had proposed. Critics say it passed despite White's leadership. "The feeling in the legislature was why should we bail out Boston to pay for Kevin's political machine," Sheriff Kearney said.

Still, the ending has been about as happy as can be expected in urban politics. Police and firefighters are back on the job, demonstrators are off the streets and 80 percent of the homeowners will receive reduced property tax bills this year.

The question is whether the saga of a canceled birthday party can have so upbeat an outcome.