On the afternoon of June 1, the newly installed public advocate, Bill de Blasio, did what public advocates of New York city do, or should at least appear to do-he listened to The People. In a uniform of pinstriped pants, red tie and white shirt, Mr de Blasio stood at the corner of Flatbush Avenue and Hanson Place, before an easel bearing the words "How can we help you," and asked passersby how he could help them.

A woman named Tina, whose head barely reached above Mr. de Blasio's belt, complained about charter schools. A retired English teacher named Zelphia Phillips complained about the new owners of Tivoli Towers. More constituents crowded in. Why couldn't the M.T.A. ever figure out its budget? How can my cell phone company bilk me as it does? Mr. de Blasio slouched the slouch of the commiserating tall man-a posture arching from his inclined head through his rounded shoulders to his bent knees. WFUV taped. Brooklyn 12 filmed.

It was a well-coordinated act of political theater by one of New York's most astute political minds. It also came off as kind of sincere.

Mr. de Blasio, 49, is something of a paradox. He's an earnest-seeming son of a labor leader who in his youth marched against nuclear power and flew to Nicaragua to protest Reagan's cold war Central American policies; a Brooklynite with a dad-from-Family Ties beard who got married in Prospect Park under a tree to an African-American woman he met in the Dinkins administration, in a ceremony that was officiated by a biracial gay Protestant couple. He is also one of New York's shrewdest, most ambitious political operators, having honed his skills behind the scenes in the service of, among others, Andrew Cuomo and Hillary Clinton.

"One of the things that I've noticed about Bill from the morning after his election was that he seemed to take very seriously the responsibility of holding a citywide office," said Ken Fisher, a former councilman. "But he's also an operative."

Or, as consultant Hank Sheinkopf put it, "He has ambition. And he's smart. And when you have ambition and smarts and desire for power together, it's a pretty potent combination."

Given that background, Mr. de Blasio has surely made the following calculation, even if he won't own up to it: Should he do a credible job as public advocate, one of only three citywide elected positions; should he carve out a starring role for himself in the theater of New York City politics; should he broaden his appeal beyond his union sponsors to encompass the city's aristocratic business and real estate firmament, he will end up among the top contenders to succeed Mayor Bloomberg in 2013.

BILL DE BLASIO'S liberal roots run deep. His mother worked in the Time magazine research department under Whittaker Chambers, and was the organizing chair of her union local. Both she and her husband, a veteran of the Battle of Okinawa, were forced to defend themselves against accusations of communist loyalties during the McCarthy era.

"It was a very searing moment in their lives, and they were entirely patriotic people," recalled Mr. de Blasio, sitting at a conference table in his blue-carpeted office on the 15th floor of One Centre Street on Monday, June 7.

Mr. de Blasio majored in urban affairs at N.Y.U.. He marched on Three Mile Island. He got a masters in international affairs at Columbia, focusing on what he thought would be the preeminent issue for decades to come.

"The United States involvement in Central America in the 1980s was horrendous and anti-democratic," said Mr. de Blasio, who distributed medical supplies in a rebel-torn Nicaragua as a member of Quest for Peace. He rejected Reagan's support of the rebel Contras, who were attempting to overthrow the socialist Sandinista government. By 1990, the Sandinstas slipped from power and Central America fell out of the headlines. Around the same time, one of Mr. de Blasio's friends got him a job on the Dinkins campaign, which, to progressives in New York City, was comparable in unvarnished idealism to Barack Obama's presidential run.

"It became a crusade," said Henry Berger, an election lawyer who's worked with Mr. de Blasio.

"It totally opened my eyes to what was possible," said Mr. de Blasio who stuck around as a senior aide.

Following Mr. Dinkins' sobering reelection loss in 1993, Mr. de Blasio's threw himself into the operative world, ultimately working for Andrew Cuomo as a political appointee in charge of HUD's operations in the metropolitan area. He went on to run Hillary Clinton's campaign for Senate seven years later. Then, in a quixotic move that surprised many observers, Mr. de Blasio ditched a seemingly limitless career in consulting, deciding instead to become a political principal himself: He ran for City Council in 2001 and won.

"I got married, started having kids, and my whole sense of life was changing around me," Mr. de Blasio said. "I really wanted to have a community-based life. I wanted to have a family-based life and be very, very involved. And it all came together very nicely with the opportunity to run for Council."

Mr. de Blasio says he intends to use his new position as public advocate-he beat Mark Green, Eric Gioia and Norman Siegel last year-to continue much of the work he began on the Council: provide a greater voice to public-school parents; reform police-community relations. He also wants to use the office to foment grass-roots activism by teaching community members how to organize; and to make government more transparent by publicly tracking both earmarks and Freedom of Information requests. He intends to do so despite the drastic budget cuts to his office-he can afford only 27 staffers, compared to Mark Green, who averaged 45.

GOING FROM RUTHLESS political operative to do-gooder elected official is not a transition that just anyone could make.

"Bluntly, some people should never try to make that leap, because you have to have a certain level of optimism, and you also have to have a lot of desire to give yourself over to people," Mr. de Blasio said. "There are a lot of people who I would term operatives who just would never stomach that. They're not bad people, but they're way too cynical."

Ironically, that's just the sort of criticism that's often leveled at Mr. de Blasio.

Critics point to his involvement with the Working Families Party, which is now under investigation by the U.S. attorney for violating campaign finance law, as an example of that cynical decision making at work. (In an undeniable demonstration of his willingness to engage, when push came to shove, in realpolitik, Mr. de Blasio won a crowded primary with help from the WFP with the full knowledge that their political arm was at least stretching the rules in helping him, and that the campaign finance authorities weren't in a position to do anything but scold retroactively.)

They also point to his wishy-washy stance on Atlantic Yards- opposed by many members of his district, yet supported by his strongest political backers, ACORN and the unions-and his flip-flopping on term-limits reform. (When he was running for speaker of the Council in 2005, he was for it; by last year he was its most vociferous opponent.)

And, most recently, they have been able to point to his statements against further taxation of Wall Street, which the New York Post lauded in an editorial: "Basically, the public advocate office is a useless waste of taxpayers' money. But if de Blasio chooses to put it to productive use, good for him."

Mr. de Blasio, naturally, dismisses the idea that his actions are driven by cold political calculation.

"I find it unbelievable," Mr. de Blasio said. "When you've already gotten a sense of the politics I've come from and the things I've done, with no self-congratulation intended, that's not the profile of someone waiting to know which way the wind blew."

drubinstein@observer.com