Interior Secretary James G. Watt so far has maintained the cautious good-will of the National Wildlife Federation, the largest environmental group, but he might not have it much longer, federation President Jay D. Hair said yesterday.

Hair told Washington Post reporters and editors that the 4.7-million-member group had sought "to be a unifying force" between Watt and conservation groups that have denounced Watt's policies. He called last month for "a kind of corporate detente" in which the federation "would sit down and see where these people are coming from."

Watt in turn has praised the group as being reasonable and representative of mainstream environmental thinking. Two-thirds of its members are Republicans, and half of its members are hunters, Hair said.

But recently the federation has begun getting "the same kind of signals the other groups are getting" from their memberships, especially ones criticizing Watt's policies on opening offshore wildlife areas to oil and gas drilling, Hair continued.

"Someday we may go in there [the wildlife areas], but let's make it the last day rather than the first day," Hair said. "We're very opposed to making them the first priority."

Hair said he could "see a psychological gap growing between the White House and Watt," and said he had been told privately that Watt did not fully represent President Reagan's thinking on land-use matters.

Watt could be perceived as a liability to any Reagan reelection campaign, Hair said, particularly since there is evidence of new opposition to Watt's policies from Indian groups, religious fundamentalists, farmers and even industrial groups that had previously backed him.

The federation is polling its members on Watt's approach and on what position Hair should take, and will announce its findings in mid-July, Hair said.

"When and if we come off the fence, it'll be with more and greater credibility than if we had attacked from the first," he said