You can look at population statistics. You can look at income figures. You can count Fortune 500 firms that have left, and you may even note a diminution in intellectual activity. But if you really want to measure New York State's free fall from preeminence, look upon the smiling Republican face of Sen. Alfonse D'Amato. That grin belongs to a man who is running for reelection virtually unopposed.

Yes, D'Amato. I am talking about a man whose name is associated with no issue, no cause, no major piece of legislation. I am talking about the man who never met a ribbon he couldn't cut, a federal grant he couldn't announce, a Social Security check he wouldn't be willing personally to deliver and who unhesitatingly plunges into any controversy, courageously putting popularity above principle no matter how many votes it earns him.

I am talking about a man who volunteered himself as a character witness for subway vigilante Bernhard Hugo Goetz, a man he had never met. I am talking about a senator who shamelessly pronounced the murdered Leon Klinghoffer a hero and recommended him for a Congressional Medal of Honor to which, as both a civilian and a victim, he was doubly not entitled. This, in all his epic opportunism, is the man who has brought the once- grand New York Democratic Party to its knees. New York's new Happy Warrior has to be Al D'Amato. He's all-but unchallenged for reelection.

Geraldine Ferraro said no. Elizabeth Holtzman said no. Arthur Levitt Jr., son of the former state comptroller, also said no but might change his mind. No one in the Democratic congressional delegation is willing, and so the only candidate is Mark Green, a 40-year-old political activist and author who has already lost one congressional race. If he's the David for this particular Goliath, he needs at least $3 million in his slingshot.

For any state the triumph of mediocrity is regrettable. But New York is not any state. Its elected officials almost automatically have become national figures, often presidents. This is a tradition that goes back to Martin Van Buren and Millard Fillmore and includes the cousins Roosevelt, the Republican Theodore and the Democratic Franklin, as well as Chester Arthur and Grover Cleveland. Even Dwight Eisenhower and Richard Nixon were Manhattan residents on their respective inauguration days.

But the political pedigree that began with Alexander Hamilton has petered out with D'Amato. He is a political mongrel -- a suburban Republican with the ethics of a Democratic ward heeler. When he was Hempstead town supervisor, he participated in a system under which public employees were coerced into contributing 1 percent of their salaries to the GOP. In 1975, he said he knew nothing about the system. In 1985, he said he did, but called it "voluntary," and then, in the manner of someone anticipating a cheap shot, he reached for one of his own. He predicted the Democrats would sink to making his integrity an election issue. He needn't have worried. A well-known opponent for One Percent Al can't even be found.

To be sure, D'Amato is a canny politician. After having won a three-way race with 45 percent of the vote, he went on to woo each and every one of New York's important special interest groups. Wall Street loves him because, aside from asking for its contributions, he has used his Senate committee assignments mostly to leave it alone. The large and powerful Jewish community loves him because D'Amato loves Israel. In fact, the leaders of this normally Democratic constituency group have tried to discourage Democrats from making the race. They like things just as they are.

And so, it seems, do some of the state's important Democratic politicians. D'Amato has been their connection to the Reagan administration -- its programs, its grants. As he says, he delivers, and he has delivered so many favors, wooed so many constituencies and accumulated so much campaign money that, according to conventional wisdom -- but not some analysts who have seen Ferraro's polls -- he's unbeatable.

D'Amato is the assemblyman as senator, a man who asks not what you can do for your country, but what small favor he can do for you. His is the triumph of special-interest politics, a pack-a-day smoker if New York grew tobacco. The dominance of Al D'Amato rebukes New York's pretensions: The Big Apple's been in the barrel too long.