What happens when the chairman of the National Endowment for the Humanities, a $130 million federal agency designed to foster scholarship and inquiry, views a television program his own agency has funded and doesn't like it?

What if he tells the newspapers it was one-sided propaganda that his grant panels should never have encouraged?

What if he calls the program "ideologically tendentious" and "'not true?"

Will his opinion and taste be taken as a new gospel, overshadowing the decision-making capacity of the bureaucracy beneath him? Will it chill the very creative climate he is empowered to nurture? Does it raise the specter of interference from the top? Of censorship?

After all, William J. Bennett, a chairman of a conservative bent appointed by President Reagan, could merely have sent someone a memo, called together a secret meeting or fumed at his water cooler.

Instead, he said publicly that he found "From the Ashes . . . Nicaragua," a documentary that aired Wednesday on public television, to be "a hymn to the Sandinistas" which ignored human rights violations in that country and was in effect "a propaganda piece, not a piece on the humanities."

The program, partly funded through the Wisconsin Committee for the Humanities, portrayed life in Nicaragua from the point of view of a shoemaker's family. The family found conditions much improved under the Marxist-Leninist Sandinista government, but feared the United States government sought the overthrow of that regime.

As a reviewer, Bennett's opinion was one of many.

Helena Solberg Ladd of Washington, who directed the film, called the attack "completely unjust," said she was surprised by the "ferocity" of Bennett's remarks and warned of "political and artistic censorship . . . every time you present a point of view contrary to the official one."

Ward Chamberlin, president of WETA, found the program "one-sided. If I were having people over for dinner, I wouldn't show it to them." But Chamberlin showed it anyway, after appending a discussion in which selected persons--including the Nicaraguan ambassador--commented to add "balance."

But, as chairman, Bennett's opinion is more than an opinion or a review. It seems to go beyond the single program as though he intended to set policy, name guidelines, influence future productions. Was he trying to send his own grant panels a message?

"If you mean am I reminding them of their guidelines, yes," he said. "I checked the legislation. As chairman, I am to assure that funds paid to grant recipients will be expended solely on programs which will carry out the objectives of the agency.

"The basic grant requirements say 'projects must broaden and deepen our understanding of the humanities' and that 'a committee cannot fund programs that seek to champion causes, or fund programs that do not strive for balanced presentation of the issues. Yes, I am reminding them of these guidelines."

The history of public television--often the vehicle for projects funded by both the arts and humanities endowments--has been a history of guidelines, and skirmishes over them.

The commercial networks early cast their lot with entertainment programming, and when PBS sought to become a public affairs oriented "fourth network" in the early 1970s, it was roundly attacked by the Nixon administration for reflecting views not in concert with those of the administration. But despite the arguments, funding continued to increase.

Now, in a time of general belt-tightening, it has been reduced. The availability of humanities endowment money is thus more sought after than before by those with potentially controversial topics.

Bennett says he is ready to speak out again. "If they're stepping into contemporary topics, they'd better be sure they have balance," he says.

But the humanities endowment is already committeed to stepping into a contemporary topic, with a 13-part series on the Vietnam war that has already been funded. Any such ambitious Vietnam project, however balanced, is likely to be controversial.

Bennett says he is reserving judgment on that program so far.

"I want to see it as soon as I can," he said. "But I don't know when that will be. The rules are that the funding agent can't see it before it goes on. I think that's to prevent censorship." Whatever else Bennett's attack may have done, it has sent the National Endowment for the Humanities and its potential grant applicants one clear message:

The chairman is watching his television.