Mood is basically upbeat in country's changing capital

Washington, in the third spring of the Reagan administration, is showing signs of confidence, maturity, renewal. Like other great world capitals - Rome and London, for example - Washington is showing a capacity to change, adjust, and to absorb the latest regime as it has earlier ones.

There are deep political differences in this town today, but little acrimony.

The lack of confidence in national leadership, which dragged at President Carter during the energy crisis of four years ago, and at other White House leaders during the Watergate decade, is little in evidence. Now the questions revolve around policy direction rather than the strength of political institutions.

The administration's antagonists, domestic and foreign, are, if anything, doing better than ever.

The Democratic National Committee has just broken ground for a new headquarters building - the Democratic Party's first ''truly permanent home'' since its founding by Thomas Jefferson in 1787. The Soviet Union, meanwhile, continues apace to finish an enormous embassy compound - a complete satellite community within the US capital district.

Liberal activist groups thrive. Americans for Democratic Action now reports 80,000 members, up 50 percent since Mr. Reagan took office. The ADA's agenda on the budget and on arms - defeating the MX, promoting the nuclear freeze, and offering liberal alternatives to Reagan defense policies - run directly counter to Reagan's. Common Cause, the progressive citizens group formed in the wake of Vietnam and Watergate, has grown in membership from 206,000 in early '82 to over 250,000 today. It emphasizes the Equal Rights Amendment, opposition to the MX missile, and promoting campaign finance reforms.

The big think tanks around the city are experiencing a burst of activism. The Brookings Institution, which helped guide the great social program expansion of the 1960s, has begun tossing around its policy weight again. Stalwarts like the Heritage Foundation and Roosevelt Center have been joined by newcomers - the Democrats' Center for National Policy and C. Fred Bergsten's Institute for International Economics.

Washington's government labor force, overall, has barely grown in a decade - despite assertions by the last four presidents of greater bureaucratic concentration here.

In fact, Washington stopped growing more than a decade ago. The metropolitan area grew from 2.1 million persons in 1960 to 2.9 million in 1970 - and has added only 150,000 since.

Even the burst in congressional staff growth was over by 1979-80, when Jimmy Carter was still president.

What's going on now is a professionalization of the capital's work force. This can be seen in many ways.

This June, some 10 percent of Harvard's Class of 1958 will make its way from the Washington area to Cambridge, Mass., for the class's 25th reunion. Psychiatrists, economists, as well as State Department staffers and journalists are represented in the Harvard class. All are part of the permanent Washington community.

National organizations are increasingly turning to Washington for their base. Some 28 percent of all national organizations are now operating here. Trade and professional groups are now the third-largest Washington industry, next to government and real estate.

Lawyers find Washington more of a haven than ever. The District of Columbia Bar Association reports its membership climbed from 35,000 to more than 38,000 the past year alone - compared with 25,000 members in 1978. Black lawyers, who have their own Washington Bar Association, increased their ranks by 200, to over 700, in the same five-year period.

Growth in the number of lobbyists, lawyers, and other professionals has taken place against an overall decline in Washington's work force of some 8,000 jobs in 1982. That was the first civilian work-force decline in the area since 1950, reports the Washington Council of Governments.

''The area effectively stopped growing in the '70s,'' says Don Starcinik, Washington-area specialist for the US Census Bureau. ''But the age distribution has shifted from a heavy concentration of children to adults.''

Physical evidence of change in the Washington area - a new modern convention center downtown and a restored Pennsylvania Avenue - reflects a urban renewal phase that took place earlier in cities like Boston and Baltimore. But the upgrading of hotels and promenades continues steadily.

The Washington establishment this spring anticipates two major events.

At the moment, the city is getting ready for the Williamsburg, Va., economic summit of Western industrial powers at the end of May. Beyond that, Washington is orienting itself for 1984 and its quadrennial emotional high, a presidential election.

Thus in the spring of 1983, Washington's mood is largely upbeat. There are relatively few public demonstrations. The White House incumbent looks increasingly comfortable with his role. And the six Democratic contenders act as though the value put on political power has certainly not depreciated.