Moving to capitalize on President Reagan's landslide victory, some fledgling Republican lobbying firms are expanding rapidly, picking up the cream of the Reagan-Bush campaign and adding Democrats to gain bipartisan leverage.

With the prospect of substantive tax legislation and deep budget cuts, two firms -- Bill Hecht and Associates and Black, Manafort and Stone -- are trying to position themselves as major legislative players.

At the same time, two proteges of retiring Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) have put together a new lobbying firm with four Republicans and four Democrats and a client list including the National Football League, the Tobacco Institute and the Outdoor Advertising Association.

Such maneuvering is natural after a presidential election, but the intense activity in Republican firms reflects the strength of the GOP and the growing belief that it no longer is a minority party.

The widespread expectation that tax and budget legislation will result in a bonanza for Washington lobbyists has provided an economic incentive for the maneuvering, taking place in every firm specializing in government relations.

"Over the next two years, there is going to be a lot of action, business is going to be good," said Robert C. McCandless, a Democratic lobbyist. "I don't think it is time to gloat. It's part of a national emergency. But I think that everybody worth his salt is going to be awfully busy trying to help in the process to make sure his clients get an equitable shake."

Some of the calculations in the lobbying community involve subtle analyses of relationships.

For example, the change in the Senate leadership under which Sen. Robert J. Dole (R-Kan.) will go from Finance Committee chairman to Senate majority leader is expected to improve the status of Charls E. Walker, a lobbyist who represents heavy industry.

Dole, according to those close to him, had little or no affection for Walker, whom he saw as a leading proponent of special-interest legislation. Dole is being replaced as Finance Committee chairman by Sen. Bob Packwood (R-Ore.).

Packwood's home state is heavily dependent on the timber industry. One of Walker's main clients in the Weyerhaeuser Co., a major timber company, which also support's Walker's American Council for Capital Formation.

Walker declined to discuss his relationship with specific senators but noted that he and Packwood are strong supporters of tax legislation designed to promote capital formation.

"I have always had the highest regard for him," Walker said. "Packwood is pro capital formation."

The lobbying community also is awaiting with interest a decision by John Salmon, chief counsel of the House Ways and Means Committee, who has given notice that he plans to join a private law firm. Salmon is known to be considering several offers, including one from Dewey, Ballantine, Bushby, Palmer and Wood.

However, the more significant activity involves the attempt of Republican firms to gain lobbying muscle. Four years ago Black, Manafort and Stone entered the fray with strong political credentials, which have grown since the last campaign:

Charles Black was political director of Reagan's 1980 campaign through the New Hampshire primary and was a senior consultant to the Reagan-Bush '84 Committee. Paul J. Manafort was a consultant and served as political director of the 1984 GOP convention. Roger Stone was the northeast regional director for the Reagan-Bush campaign.

The political consulting arm of the firm has added partner Lee Atwater, deputy manager and political director of the Reagan-Bush '84 Committee. Atwater said he expects to work for the separate lobbying arm of the firm but will do so on retainer, rather than becoming a partner of that arm.

On the lobbying side, Black, Manafort and Stone hired two Democratic specialists in campaign contributions, Peter G. Kelly, finance chairman of the Democratic National Committee, and James C. Healey Jr., an aide to Ways and Means Chairman Dan Rostenkowski (D-Ill.) who specialized in raising money for Rostenkowski.

"My role will be to provide them with a Democratic presence, particularly on the House side," said Healey, who also acted as Rostenkowski's liaison to the House Democratic leadership.

Among the firm's recent clients are Suburban Propane Gas Corp., Salomon Brothers, Inc., the Solar Energy Industries Association, American Petrofina Inc. and the National Securities Clearing Corp.

At the Capitol Hill offices of Hecht and Associates, three new people have been hired. Two are from the Reagan-Bush campaign -- Edward J. Rollins, the campaign manager and former White House assistant for political affairs, and Craig Helsing, deputy to Atwater at the campaign and now finance director of the Innaugural Committee -- and one from the Walter F. Mondale's campaign, budget director Frank West.

"It's kind of a whole new thing for me," Rollins said. "I'll be working for corporate-type clients who need access to people in government. I know an awful lot of people in government."

Rollins said he plans to work directly in politics in the 1986 and 1988 campaigns, separate from his work for Hecht and Associates.

Hecht said that Stuart Spencer, a key Reagan campaign adviser and co-founder of the firm, will lobby more actively in Washington during the second Reagan term. Spencer runs a political consulting firm in California.

Clients of the Hecht firm include the American Security Council, the Los Angeles Raiders, South Africa, Brown and Williamson Industries and the Irvine Corp.

Last August, two former Baker aides, Marty Gold, who had been counsel to the majority leader, and Howard S. Liebengood, former Senate sergeant at arms, formed Gold and Liebengood. Since the election, the outfit has put together an eight-member organization split evenly between Republicans and Democrats and between House and Senate specialists.

On the Republican side, in addition to Liebengood and Gold, William F. Hillenbrand, former secretary of the Senate, and John Scruggs, former White House lobbyist, have joined the firm.

On the Democratic side, two former House members, Ray Kogovsek (Colo.), who retired, and William R. Ratchford (Conn.), who was defeated, have been hired as have Denise Bode, former legal cousel to Sen. David L. Boren (D-Okla.), and Chuck Merin, a Democrat who had worked for the Tobacco Institute.