John B. Anderson yesterday said he is forming a third political party as a vehicle for a 1984 presidential bid because the two major parties are "captives of special interests" and "unwilling to experiment with new ideas."

Anderson said he and a group of supporters are drafting a set of principles for the new party and that he hopes to get the party off the ground by next fall.

The former Republican congressman from Illinois said he doesn't think a third-party candidate can win the presidency in 1984.

"The emphasis has to be on establishing a new national debate whether you win the election or not," he said in an interview in his Georgetown office. "I've told people they have to be prepared for the long haul."

Anderson, 61, won 6.7 percent of the vote as an independent presidential candidate in 1980.

He is tanned, relaxed and 12 pounds lighter than in 1980, when he ran as a dark-horse presidential candidate, first as a Republican and then as an independent. He said he has spent much of the last 2 1/2 years speaking around the country--at $2,000 per appearance--and pondering another run for the presidency.

He conceded that there is no obvious groundswell of public interest in a third party, but said that "people are very frustrated" with the two-party system.

"There is that vague feeling out there. How you kindle that into a flame? I don't know," he said. "But someone has to try. It has to be a leap of faith.

"Somebody ought to break the gridlock. We're the only western democracy that doesn't have a viable third party."

Anderson would have one big advantage in running as a third-party candidate--a $6 million bankroll. The Federal Election Commission has said he probably would be eligible for about that much in public funds in 1984 if he forms a third party because of the support he won in 1980.

He insists that this is a "modest sum" by national political standards, and the subsidy isn't motivating his third-party effort. "I don't feel like the $6 million bionic man," he added with a grin.

Though no third-party presidential candidacy has succeeded in the United States, polls indicate that there is a great deal of interest in creating a third or even fourth or fifth party.

In November, 1981, 49 percent of the respondents to a Washington Post-ABC News poll said they would like to see a new party formed. In September, 1982, the number had dropped slightly to 44 percent.

Anderson moved his offices from his home into a small, sparsely furnished suite on the edge of Georgetown in March. Old campaign posters hang on the walls, along with snapshots of former aides.

He has a staff of three in the new offices, and puts out a periodic newsletter, called "An Independent View," to inform supporters about his activities.

Many of Anderson's top former aides privately oppose his making another presidential race, but he recently wrote to 40,000 former contributors seeking their views. He said 68 percent of those who replied "would like to see another campaign," 48 percent "want a new party" and about 15 percent don't want him to do anything.

The decision to go ahead with a third party was "an evolutionary one," he said. "We have a very mature political system, and our two political parties have stopped being innovative, they are unwilling to experiment with new ideas . . . . The two parties are being ruled by the very interests they are supposed to mediate, they've become captive of special interests."

He said he and a number of supporters have begun writing a set of principles that would form the basis of a platform for the new party. He envisions producing a document of 5,000 to l0,000 words by September that will make the case for a new party.

He said that, among other things, the platform would call for "a new concept of national defense," creation of a public enterprise corporation to ease the nation's long-term unemployment problems, "pay-as-you-go government" and a reform of the political nominating processes.

What would the party be called?

Anderson and his advisers have kicked around a number of names, including New Party, Independent Party and Independent Unity Party. But for now the name on the door of his offices is the one he used in 1980: the National Unity Campaign.