Three years ago, after Daniel Patrick Moynihan had announced his intention to retire from the Senate after 24 years, Peggy Breen, a prominent Manhattan preservationist, began closing all of her letters to the senator with the same question: "What are we going to do without you?"

It is a question many in the worlds of architecture, urban planning and preservation can ask with rueful finality now, for Moynihan died yesterday in a Washington hospital at age 76, of complications from an operation earlier this month.

Moynihan will, of course, be remembered for many achievements and involvements. He was a skilled politician and an accomplished scholar, an independent thinker, a gifted writer and a speaker of serious wit.

But throughout his varied career, his interest in cities and architecture was a constant theme. Early on, he became absorbed with the idea of using political power to get things built, and to make sure that what did get built in the name of all of us was as good as it could possibly be. Conversely, when a good building was threatened with destruction and he could do something about it, he did.

It would be hard to overstate the depth and breadth of Moynihan's involvement in these fields, or his effectiveness. He took on incredibly complex projects, such as the highly inventive, still-in-process reconfiguration of New York's Penn Station, and pushed them with relentless will and authority. (Not incidentally, perhaps, Penn Station was one of the buildings Pat Moynihan had often scurried through in his hardscrabble New York youth.)

What to Moynihan seemed common sense often ran against the grain of conventional wisdom. In Buffalo, he noticed the sorry state of Louis Sullivan's 1896 Guaranty Building -- one of the triumphant expressions of American architecture. The mayor of Buffalo was incredulous when Moynihan announced his intention to save the building, which he promptly did. And, after securing federal funds to help with the restoration, Moynihan moved his little Upstate office into the structure.

No city has benefited more from Moynihan's architectural interests and savvy than Washington, which, on and off, was his second residence for more than four decades. He helped get a lot of things built -- the revived Pennsylvania Avenue, among them -- and, of late, he showed both his intelligence and courage by campaigning against the uglification of our beautiful capital in the name of security.

As a young aide in the Labor Department during the Kennedy administration, he turned what many would have treated as a routine assignment -- writing a report for something called the Ad Hoc Committee on Federal Office Space -- into an opportunity to establish "Guiding Principles for Federal Architecture."

"The belief that good design is optional, or in some way separate from the question of the provision of office space itself, does not bear scrutiny," Moynihan wrote with characteristic vigor. Though the guidelines often have been ignored, their very existence has been important -- the present-day Design Excellence program of the General Services Administration owes much to Moynihan's example.

That long-ago report also proved to be the beginning of the rejuvenation of Pennsylvania Avenue. For decades President Kennedy was credited with the idea -- during his inaugural parade he had been embarrassed, it was often said, by the sad disrepair of the mostly 19th-century buildings on the avenue's north side. Actually, it was Labor Secretary Arthur J. Goldberg who was taken aback by the avenue's dilapidated condition. "The touch of Goldberg's genius was to propose the rebirth of Pennsylvania Avenue as part of an otherwise routine procurement project," Moynihan said.

Shortly after the report was published in 1962, however, the president appointed Goldberg to the Supreme Court. It then fell to Moynihan to see the vast project through, which he did, with a little bit of luck, from beginning to end.

In the early years, he became the chief shepherd to Nathaniel Owings, the architect Kennedy had chosen to envision the first big plan for Pennsylvania Avenue. Moynihan loved to recall the many evening strolls he took with Owings along the avenue. They'd often pause in front of the National Archives. "We would sit on those nice strong benches," Moynihan recalled in an interview, "and look up at the Old Patent Office and see Rome or Italian hill towns."

Moynihan later would admit he didn't particularly care for the grandiosity of that first Owings plan, but he was genuinely moved by the older man's boundless enthusiasm for the project. As it happened, Moynihan had an opportunity to help change the plan for the better when, during a brief Republican interlude, he joined the Nixon administration and in the early 1970s helped shape the Pennsylvania Avenue Development Corp., the powerful agency that finally took on the monumental project.

Fate seemed to tie Moynihan to the avenue's future. Because of a real estate recession, nothing much happened with the PADC during the 1970s. By the time things did start to move, Moynihan was ensconced on Capitol Hill as a Democratic senator from New York. Throughout his Senate career he was the avenue plan's most powerful ally in Congress, and completing the Federal Triangle with the Ronald Reagan Building and International Trade Center in the mid-1990s was largely due to his interest and influence.

Not surprisingly, as soon as the apartments atop the new Market Square buildings at Pennsylvania Avenue between Seventh and Ninth streets became available about a decade ago, Moynihan moved into one of them with Elizabeth, his wife and longtime political adviser. In the mornings, the pair could step out on the balcony for a striking view of the Capitol. How fitting.

Moynihan was fond of quoting Thomas Jefferson to the effect that "design activity and political thought are indivisible." He profoundly believed that, and as a nation and a city we profited greatly from this belief.

Throughout his career, Daniel Patrick Moynihan weighed in on the power of architecture and urban design.During his tenure in the Nixon administration, Daniel Patrick Moynihan helped shape the Pennsylvania Avenue Development Corp., which oversaw the street's rebirth.