The Reagan administration returned to Washington without Ronald Reagan yesterday. The official reason was to dedicate a new building in his name. But in public addresses, private remarks and glitzy party chatter, Republicans and Democrats alike seemed more eager to recharge themselves on the energy of days not so long ago when simple truths and unvarnished optimism were fashionable for a time.

They traded favorite quotes from the Gipper. They did gentle impersonations of that unforgettably soft voice. And yes, they argued over the meaning of his legacy.

The Nostalgic in Chief was President Clinton as he joined former first lady Nancy Reagan for the dedication ceremony in the soaring atrium of the Ronald Reagan Building and International Trade Center at 1300 Pennsylvania Ave. NW.

"If you look at this atrium, I think we feel the essence of his presence," Clinton said. "His unflagging optimism, his proud patriotism, his unabashed faith in the American people. I think every American who walks through this incredible space and lifts his or her eyes to the sky will feel that." Nancy Reagan, who entered and exited the ceremony on Clinton's arm, said she wished her husband, who has Alzheimer's disease, could have been present. "Ronnie would certainly be very pleased that a monument to him embraces his lifelong belief in the importance of a national commitment to free and open trade," she said.

She received two standing ovations from the audience of several hundred past and present government officials.

On the dais with Clinton and Reagan was a smattering of faces from two administrations and two political parties. There were former secretary of state George P. Shultz, former senator Robert J. Dole, retired Gen. Colin L. Powell, Republicans Strom Thurmond, Trent Lott and Tom DeLay and Democrats Charles S. Robb and Steny H. Hoyer, among others.

In the evening, Reagan admirers and former political foes gathered in the same atrium, made of what architects calculated was an "acre of glass," for a reception, dinner and concert sponsored by the Reagan Foundation.

The new building that bears Reagan's name is a massive edifice that completes a century-old dream of presidents and planners for a corridor of grand architecture along Pennsylvania Avenue.

On the site of an old parking lot that President John F. Kennedy noted for its "surpassing ugliness" now rises the second-largest federal office building, after the Pentagon, a new home of cut limestone and polished granite for 7,000 government employees and private office workers.

But the monument to the president who extolled lean government was not without its budget problems.

It cost about $818 million, more than double the original estimates received by Congress. It was completed four years behind the original schedule. Federal employees started moving in last summer, and a little construction work remains to be done.

The trade center portion of the Reagan Building will promote U.S. exports and serve as a focus for trade programs, conferences and trade-related private companies. The federal office part of the building will contain offices for the Agency for International Development, the Customs Service and the Environmental Protection Agency.

Reagan signed legislation to start the project in 1987.

"It was only right that the grand building itself be named for President Reagan, one of whose many gifts to the American nation is a vision of a world at peace and free to trade," said Sen. Daniel Patrick Moynihan (D-N.Y.), who sponsored the original legislation and who has been a dogged advocate of redeveloping Pennsylvania Avenue since the Kennedy administration.

The building also will house the Woodrow Wilson International Center for Scholars, and Clinton linked Wilson and Reagan in his remarks as two globally oriented presidents whose legacies "span much of what has become the American century."

Then Clinton invoked Reagan to tweak Congress into supporting his foreign policy initiatives, such as funding for the International Monetary Fund and the United Nations.

"President Reagan once said we had made what he called an unbreakable commitment to the IMF," Clinton said.

City officials are counting on the Reagan Building to help lift the District's economy. Its public arcades and courtyards are designed to draw people up from the Mall to downtown. It is expected to provide up to $45 million a year in revenue from visitors attending conferences at the center.

"This building is as grand as its purpose," said Del. Eleanor Holmes Norton (D-D.C.). "It is a place to help make the country grow strong and the city stand tall."

Federal employees who have moved into the building offered rave reviews.

"Watching this place materialize from a lot of dirt and construction, it's almost been like magic," said Verla Sutton-Busby, a program analyst with the EPA.

About 1,000 of the agency's employees have moved to the building, where they have ergonomic amenities such as more work space and better lighting, and they feel a stronger connection to downtown Washington. At the turn of the century, the site of the Reagan Building in the Federal Triangle between Pennsylvania Avenue and the Mall was the city's red light district. Planners have been seeking to redevelop the area for nearly 100 years.

As Moynihan recalled yesterday, Kennedy was appalled by the sorry state of Pennsylvania Avenue during his inaugural parade 37 years ago and called for revitalization of the area.

With the opening of the Reagan Building, the last of the blight that Kennedy saw is gone. Word has traveled fast among the federal work force. The Reagan Building is now one of the most desirable places to be assigned. Friends are calling up and asking for tours, Sutton-Busby said. CAPTION: Nancy Reagan acknowledges Sen. Daniel Patrick Moynihan, sponsor of the original legislation for the building. CAPTION: President Clinton escorts the former first lady. CAPTION: An atrium soars over those attending the Reagan Building dedication, including former first lady Nancy Reagan at the podium, left and above. "Ronnie would certainly be very pleased that a monument to him embraces" his commitment to free trade, she said.