Among the biggest legal developments of the past decade is the U.S. Supreme Court's weakening of federal power in favor of states' rights. And no politician has taken greater tactical advantage of this new federalism than Eliot Spitzer, the attorney general of New York State.

Since taking office in 1999, Spitzer has inserted himself into areas that once would certainly have been seen as the province of the federal government. First and foremost has been his investigation of conflicts of interest among securities traders, which has already led to the extraction of a $100 million fine from Merrill Lynch in May. The firm agreed to stop its practice of routinely touting the stock of companies that also happened to be clients of its investment banking services.

To make his case, Spitzer drew on a previously obscure state law that predated by 13 years the creation of the federal Securities and Exchange Commission. One of the things that makes Spitzer such a successful lawyer is his ability to win or settle cases built on novel grounds. "My sense of him is that he's expanded the purview of the office," says Helen Desfosses, president of the Albany Common Council and a public policy dean at the State University of New York. "He's now speaking in arenas that, at least in my recollection, attorneys general haven't spoken in before."

Attorneys general in New York have, at least since the 1950s, traditionally been advocates for consumer protection, leaving criminal prosecution to district attorneys. Spitzer's predecessor, Dennis Vacco, operated as a sort of super D.A., prosecuting kiddie-porn purveyors and members of the mob. Spitzer, by contrast, is inclined to join the populist fight, wherever it is found.

He has a way of reading the prevailing political winds and hoisting his sail to catch them, pursuing high-profile cases in the areas of abortion rights, racial profiling and regulation of the charitable donations that flooded into the state following last September's terrorist attacks. Spitzer won headlines by forcing major grocery chains to pay immigrants the minimum wage and shamed the U.S. Environmental Protection Agency into taking action against Midwestern power plants he was suing for violating the federal Clean Air Act.

Not surprisingly, the Bush administration was less-than-enthusiastic about that case, and Spitzer's attempt (under an unusual interpretation of "public nuisance" law) to block gun manufacturers from supplying dealers linked to weapons used in crimes has failed to fly in court. But Spitzer has won more than his share of fights and draws widespread praise for the professionalism of his staff.

Spitzer, 43, is a native of the Bronx and was educated at Princeton and Harvard Law School. He first won fame as an assistant district attorney prosecuting members of the Gambino crime family. In the 1994 Democratic primary for attorney general, he finished last in a field of four. In the general election four years later, he beat Vacco by so few votes that the tally was recounted for weeks. (Spitzer spent $10 million borrowed from his developer father in the two races, most of which he has repaid.) This year, as he seeks a second term, he is a shoo-in for reelection against a little-known Republican judge. And Spitzer is universally considered a certain candidate for higher office in the future.

For all his ambition and skill at putting together press events, Spitzer is generally respected as smart, quick, articulate and savvy, tempering his activist reading of the law with a sense of what is politically doable--especially when his leading client is a governor from the other party. But his sense of what is doable remains expansive. "As the Congress and courts have succeeded in forging a new federalism," he said recently, "they have created an opportunity to accomplish things previously thought unsuited to state initiative."