Some of the nation's top law firms, many of which had shunned a Washington practice because they thought it involved more politics than law, now are opening full branches here.

These include some of New York's largest and most prestigious Wall Street firms such as Sullivan & Cromwell, and Dewey. Ballantine, Bushby, Palmer & Wood. Once, they dealt with the government by shuttling lawyers in and out of Washington on specific assignments.

Veteran lawyers compare the new trend to the explosive growth of the Washington law practice during the 1930s and 1940s when the New Deal and Fair Deal legislation of Franklin D. Rossevelt and Harry S Truman greatly increased the government's control of business.

New legislation affecting energy use, consumer safety, environmental protection, equal employment opportunity and occupational safety and health now mean increased government influence on the nation's businessmen.

"Government regulations used to affect only the General Motors of the country. But now they affect medium-sized businesses in Kalamazoo," said Lewis A. Engman former Federal Trade Commission chairman who recently opened a three-lawyer office for the Grand Rapids law firms. Warner, Norcross & Judd, the fourth largest in Michigan.

Engman's view was echoed by Felix B. Laughlin, who is expanding Dewey, Ballantine's two-lawyer Washington office into a full branch that will include two well-known names from the Ford administration - Philip W. Buchen, a smalltown Michigan lawyer until President Ford named him White House counsel, and Lawrence Silberman, former deputy attorney general and ambassdor to Yugoslavia.

"Our clients need lawyers here on the scene. So much is happening here," Laughlin said.

Lawyers coming here from across the country are entering one of the country's busiest and richest legal markets, one that is expanding rapidly.

With its corporate headquarters, banks and stock exchanges, New York City remains the nation's legal capital with 40,000 of the country's 445,000 lawyers. But Washington, about one-tenth the size of New York, has 20,000 lawyers practicing in and out of government, an increase of about 5,000 lawyers in the last 10 years.

Anthony Nigro, secretary to the D.C. Court of Appeals' committee on admissions, reported an increase in the number of lawyers from other parts of the country who are admitted to practice here from 1,150 and 1975 to 1,400 last year.

Traditionally, New York lawyers have looked down on a Washington practice, feeling it involves more politics than law.

Cutting through the jungle of the federal bureaucracy is an important part of a Washington lawyer's job and even New York firms with strong Washington connections now want their own lawyers based here.

"We have been doing a lot of Washington business either by phone or by shuttle," said Michael M. Maney, a partner in the month-old Washington office of Sullivan & Cromwell. That firm once was considered the New York branch of the State Department with both Allen and John Foster Dulles as partners.

"But the government is evolving so rapidly in so many new areas that there's a value in being on the spot," Maney said. "It used to be that New York was where the business was, and Washington was hwere the politics was. Now business is coming to Washington.

Shea, Gould, Climeiko & Casey, another large New York firm, has hired a well-known Washington figure in Welbur Mills, former chairman of the tax-writing House Ways and Means Committe, as a consultant on tax matters in its newly'opened Washington office.

The office was opened in the spring, and Mills was added this summer. Although he is a member of the Arkansas bar, he has not been admitted to the District of Columbia bar, a necessity to practice law in an office here.

Casey, Lane & Mittendorf, another New York firm, has expanded its office here from what used to be "an errand running shop" into a real law firm, said Henry B. Taliaferro, the partner here.

"We decided we need people on the scene who understand the federal government as it gets deeper into business affairs," he said. "We not only have new agencies (regulating business), but we have the old agencies increasing their jurisdation."

The greatest expansion into the Washington legal market has come from Houston firms that have clients in the heavily regulated energy field.

One firm, Fulbright & Jaworski, has been in Washington for 25 years, but three others have opened offices here within the past five years.

Bracewell & Patterson started 2 1/2 years ago with one Washington partner - M.R. (Duke) Ligon, former assistant administration - and now has enough business here to keep 14 lawyers busy.

Similarly, Robert Gooch, former general counsel of the Federal Power Commission, was the only lawyer when Baker & Botts opened its Washington branch in

Now there are 11 lawyers in the office.

"That's pretty much what we planned on. We expected to grow, and we expect to keep on growing," Gooch said.

Now two more Houston firms are opening Washington offices. Butler, Binion, Rice, Cook & Knapp has brought on Stephen M. Minikes, former senior vice president of the Export-Import Bank, as a Washington partner and will send a Houston partner anf three other lawyers for the new office.

"We found ourselves with five or six lawyers in Washington every week. We may as well have an office there," said Steven C. Oaks, a Houston partner who has been working to set up the Butler, Binion office here.

"Every business client of any significant size has regulatory problems. You simply can't commute every day to Washington," he added.

Another big Houston firm, Andrews, Kurth, Campbell & Jones will open its Washington office in the dall with three experienced Washington lawyers, including Michael F. Butler, former chief counsel of the Federal Energy Administration, and three lawyers from the main office.

Two large Los Angeles firms have opened offices here. The first was Omelveny & Myers, which set up a branch office 16 months ago "as a result of the realization that the federal government is here to stay and that it is having a profound impact on many of our clients," said Richard C. Warmer, managing partner here.

"We felt we could do a better job by being represented in Washington," he added. "There's more of a need for us than for (New York) firms that could get here on a shuttle."

The firm has nine lawyers here and had just added here former Secretary of Transportation William T. Coleman Jr., formerly a Philadelphia lawyer.

Gerald L. Parksy, former assistant secretary of the Treasury Department, is opening the Washington office for another large Los Angeles firm, Gibson, Dunn & Crutcher.

Parksy, who traveled around the world in his Treasury job, said he wants to use the Washington office to build an international law practice for his firm and take care of the Washington problems of present clients.

"The basic reason foe opening the office is to internationalize the firm," he said.

The Chicago law firm of Lord, Bissell & Brook, on the other hand, opened its Washington office two years ago to service one major client - Amtrak. A specialist in railway injury cases, Lord Bissell & Brooks was asked by Amtrak to help with some of its trail work, said firm chairman John Smith.

"Other than Amtrak," he added, "we wouldn't have gotten in there."

The medium-sized Philadelphia firm of Obermayer, Redman, Maxwell & Hippel opened its Washington office for the traditional reasons - to take care of existing clients who need a Washington lawyer and to grow in ways it cannot with a Philadelphia base.

Two of the four Washington partners are well known in Washington - former Senate Minority Leader Hugh Scott and former Army General Counsel Charles Ablard.