On the fourth floor of the town hall here, in a paneled office cluttered with honorary fire chief helmets and other conventional mementos of small town politics, sits Alfonse Marcello D'Amato.

Ridiculed just months ago as "Al Who?" and tagged with the nicknames "Sen. Fonzie" and "Tippy," he is preparing to pack up and move to Washington, where among other duties, he will chair the Senate Appropriations Subcommittee on the District of Columbia.

For the last three years, D'Amato, a tough-talking conservative, has served as the presiding supervisor of this amorphus Long Island community which, with a population of 800,000, is the nation's largest town. And from a town hall that looks like it belongs in Rockville, Md., Senator-elect D'Amato has helped run the nation's most rigid suburban political orgainization.

It is too soon to know what D'Amato's selection as chairman of the little-desired subcommittee will mean to District of Columbia residents. While he embraces conventional, conservative positions, opposing gun control (one of his first hires for his Senate office is a former official of the National Rifle Association), abortion and the Equal Rights Amendment, and favoring capital punishment and increased defense spending -- D'Amato sees those as national issues. "I have no deep philosophical view about someone else's bailiwick," he said.

On the surface D'Amato is the archetype of local politicians across America who have blocked ratification of the constitutional amendment that would give the District full voting representation in Congress. They are, like D'Amato, invariably white, conservative, suburban or rural Republicans who, in the phrase of D.C. Del. Walter E. Fauntroy, find that D.C. residents are "too four," that is, too black, too liberal, too urban and too Democratic.

In a campaign that brought out the worst in rhetoric, D'Amato was accused of being crooked, racist, and stupid, among other traits. One critic insists that "no one should believe that Al is dumb."

D'Amato admits that the three-way race involving incumbent Republican Sen. Jacob Javits, Democrat Rep. Elizabeth Holtzman and him was "a dirty campaign." But in an interview half a world away from here, in a midtown Manhattan skyscraper, the senator-elect, puffing hard of a dollar cigar, sought to allay fears that "folks down there" in the District of Columbia might mistakenly have about him, "based on how the media portrayed me."

What the media said, among other things, is that D'Amato and his town hall cronies catered to the fears of their overwhelmingly white (95 percent of the 1.8 million residents of Nassau County), substantially Italian Catholic, blue-collar constituents, and blamed Democratic liberals for the "welfare Cadillac" that is robbing them of their money and safety.

D'Amato, a 43-year-old second-generation American, whose trim figure and three-piece pin-striped suits belie the "pasta and polyester" tag put on him by one columnist, does not yet understand the arcane intricacies of the Congress, let alone its love-hate relationship with the Capitol City. He said he has no preconceived ideas about D.C. government.

He promised that he and his staff "will be open to the facts as presented." While he is committed to fiscal conservativism, he said that as a result of serving as presiding supervisor he is aware of the pressing problems of urban areas, particulary in the Northeast. He guessed that his experience in Hempstead will make him "sympatico, if anything," to the District of Columbia's problems.

The refined, moderate, estabishment figures in both the Republican and Democratic parties shuddered at the thought of the rough-hewn, right-wing D'Amato representing the Empire State in the United States Senate. They suggested -- when they didn't come right out and say it -- that he was way out of his league, even in these inflationary times; a two-bit politician, pulling shady deals and shaking down people for favors, and ridiculing his esteemed liberal opponents as the "ultra-liberal Tweedledum and Tweedledee."

During the primary campaign, D'Amato unmercifully bludgeoned the 76-year-old Javits as being too old and too sick. But after he pasta-pate race, D'Amato took it easy on the old man during the general election campaign, rightly figuring that Javits, running as a third-party candidate on the Liberal ticket, would pull enough votes away from Democrat Holtzman to propel D'Amato into the Senate.

Since the election, he has shown himself to be more pragmatic than dogmatic, having made peace overtures tothe state's two top Democrats, Gov. Hugh Carey and New York City Mayor Ed Koch.

Leaning back in a swivel chair, in a 22nd floor office loaned to him by Sen. David Patrick Monyihan (D-N.Y.), the successful nominee of the Republican, Conservative and Right-to-Life Parties commiserated on the telephone with a priest.

"Yeah Father," he said, "I know their perception is that I'm an obscure little local bumpkin. That's the way the media portrayed me. But did I fan the fires of biotry? They can't point to anything. But thank God, Father, we got a system like that [press freedom] in this country. . .Sure, and you have a holy, blessed Christmas too, Father."

Hanging up, D'Amato winked at a visitor and blew smoke toward an antilitter poster that implored "Don't Dump on New York," and then watched as the smoke combined with the haze outside to cloud his view of the Pan-Am Building across the street.

D'Amato said he will commute, at least until next fall, between a Capitol Hill apartment and Island Park, the working-class South Shore town that is home to a D'Amato clan that includes his wife, Penny, who is studying for a master's degree in computer science, four school-age children, parents, a brother (a state assemblyman), aunts, uncles and cousins.

During 10 years in local elective office. D'Amato developed a reputation as a smart, savy politician. Along with his friend, Joseph M. Margiotta, the GOP chairman of Nassau County, he ran a highly politicized local government, which, in the style of old-time bosses, rewarded its friends and punished its enemies. The "Village Voice" described the Nassau County GOP machine as "the shame of the suburbs."

Until a vigorous Democratic district attorney stepped in five years ago, holders of 30,000 municipal and county patronage jobs systematically kicked back 1 percent of their pay to the party. The toll currently exacted is not clear, but job holders still join the party; the town's insurance business includes annual commissions to the faithful, including $3,000 to D'Amato's father, who also has a $33,900 a year public job that seldom requires his attention, according to accounts in New York newspapers.

After Margiotta was indicated this month on charges of extortion and mail fraud, D'Amato spoke at a $500-a-person defense fund rally, calling Margiotta "a dear friend and a man who has never broken the law for himself or anyone else." The senator-elect drew an ovation with the observation that "if it wasn't for Joe Margiotta, so many of us in office today would not have had the opportunity -- and that includes Al D'Amato."

Another charge, which D'Amato denies loudly (the way he denies most charges) is that the few blacks and Hispanics who have managed to find a place to live in Hempstead have been denied use of the town-owned Malibu and Sands beach clubs, which feature saunas, chandeliered dining rooms and catered bar service. (Amenities at the town's third beach, open to all, are limited to the traditional hot dog stands and rental umbrellas.)

D'Amato offered an explanation familar to Washington football fans who have tried to get Redskin tickets: The several thousand memberships are rented annually, and there is little turnover. As close to an admission of favoritism as D'Amato will come is to admit that the slots "turn over about once every seven years . . . since political preference no longer exists."

When D'Amato is asked his attitude toward minorities, he responds obliquely that as a son of Italian immigrants, "I know about discrimination. I have experienced it," citing his rejection for employment by silk stocking Wall Street law firms.

Also, as with other Italian-Americans in public life, D'Amato has had to defend himself from rumors of Mafia connections. "It's repugnant to me, but you have to live with it." Federal prosecutors, he said, "All know that Al D'Amato has no connection with organized crime."

It was the absence of standards, he explained, that led him to fight a Justice Department order imposing hiring quotas on Nassau's 3,472-member police force -- seventh biggest in the country -- which had only 35 women (1%) and 81 minorities (2%).

His opposition to low-income housing in Hempstead, which has neighborhoods that run the gamut from the original Levittown to posh shorefront mansions, grows out of a belief that government "shouldn't force any projects, including luxury ones, where there is not community support."

Recently, he claimed that his administration broke a logjam between HUD and the town fathers, to permit senior citizens and a limited number of low-income units to be constructed in Hempstead. Precisely how many low-income units? "For 20 families, he replied.

His only standards, he said, are "ability, not the color of skin or whether you are a man or woman." One need look no further than his outer office, D'Amato said, to understand his personal commitment. "Who do you see working for me?" he asked, referring to a black woman and an Oriental male.

The interview over, D'Amato punched his intercom and bellowed, "Donna girl, will you get my wife on the phone?"