An environmental group shouldn't meddle in national politics - regardless of its attitude toward President Reagan and former Interior Secretary James G. Watt.
So suggests a leading conservationist, Russell Train, president of the World Wildlife Fund and former chief of the Environmental Protection Agency.
''I would be wary if I were an organization that had built a membership on attacking Watt because that's a fairly narrow base,'' Mr. Train cautioned in an interview here shortly before Mr. Watt tendered his resignation to President Reagan. ''If Watt disappears . . . will your membership disappear as well?''
Train contends that both sides - environmentalists and the Reagan administration - have made ''mistakes'' in their relationships to each other since the President took office in 1981.
Most environmental groups ''sort of took the oath of allegiance to Jimmy Carter'' when Mr. Carter sought reelection to the presidency, Train says. ''So Reagan comes in, and his reaction to the environmentalists (is), 'They fought me tooth and nail; I don't owe them anything.' I think that was a mistake on the environmental community's part.''
Likewise, he says he thinks the President erred in appointing ''to about every important environmental or resource management post someone who had come out of an exploitive industry or a regulated industry.''
Although he is a Republican and was a member of Reagan's advisory task force on environmental matters in 1980, Train calls the President's environmental record ''very poor'' and characterized by ''disinterest.''
''My biggest complaint has been polarization'' on such issues as acid rain, endangered species, and public lands, Train says. ''The only way, it seems to me , in our society you're going to get them resolved in an acceptable fashion is by trying to bring people together and find some common ground.
''But once we start - certainly in a presidential campaign - taking our publicly supported, nonprofit organizations into the political arena, I think it's asking for trouble.''
However, there already are indications that the environmental community is gearing up to do just that again next year.
Early in July, the Sierra Club sponsored a four-day ''international assembly'' that explored the possibility of forming coalitions with organized labor to influence the 1984 elections. The invited guests included Democratic presidential hopefuls Walter Mondale, Reubin Askew, and Alan Cranston, Gary Hart , John Glenn, and Ernest Hollings. The lone Republican of note was Sen. Robert Stafford of Vermont, who has often split with Reagan on environmental issues.
''It's time,'' Sierra Club president Denny Shaffer told the nearly 1,000 people attending, ''for the politics of conservation to move forward. And we're the right people with the right resources at the right time.''
There is no question, says Train, that the environmental movement has ''developed and demonstrated quite a bit of political muscle. . . . Everybody recognizes the environmental lobby has become a very potent lobby.
''Most environmental organizations have been experiencing healthy growth - in income and membership both. How much of this is due to Watt, I just don't know. We haven't gotten into that ourselves, but we may be the beneficiary of it.''