President Reagan will propose 50 percent cuts in the fiscal 1982 budgets of both the National Endowment for the Arts and the National Endowment for the Humanities when he submits his revised budget to Congress.

"It'll be difficult," said NEH chairman Joseph Duffey. "Very difficult."

Both the NEA and the NEH were informed of Reagan's budget decision yesterday. The NEH budget to be submitted to Congress will go from the figure of $169 million proposed by former president Carter to $85 million. tThe NEA budget will go from $175 million proposed by Carter to $88 million.

"It's damnable, absolutely damnable," said actor and singer Theodore Bikel, a member of the National Council on the Arts, the advisory body to the NEA. "It's insupportable and it's punitive. The next battleground is, of course, the Congress. We'll have to marshal all the forces we can to intercede. I've already called Issac Stern and Sidney Poitier about sending telegrams to the White House. Now they'll have to send them to the Congress."

Most observers are more optimistic about the funding picture in Congress.

"Assuming it's a 50 percent," said Rep. Sidney Yates (D-Ill.), who had not been notified of the budget figure yet, "it's too drastic. That may be Mr. [OMB director David] Stockman's priority, but it's not the Congress'. I don't think the Congress will accept such a low priority for the arts and the humanities. Mr. Stockman says that the Endowments have hindered business from contributing to arts organizations. He doesn't understand how it's worked in the past. The Endowments have been the trailblazers for contributions from business, not the other way around. I think if the Endowment cuts its contributions, so will business."

Another member of Congress agreed that 50 percent cuts would not go through Congress. "There will be a number of us suggesting smaller cuts -- like 20 percent. And I think there will not be strong opposition to that."

But first the Endowments will have to recalculate their budgets according to the Reagan administration's submission. "It's much too early to say exactly what we will cut," said Duffey. "The one thing that there is not a likelihood of is across-the-board cutting. That wouldn't be responsible. There is a possibility that some programs will be totally eliminated."

Duffey has asked his staff to look at "what they could pare, and administrative implications. For instance, if you're cutting a program in half, you have to ask is it worth continuing the program at all -- or should the money be put into something else."

The National Council on the Arts has already discussed in broad terms possible scenarios for the Endowment if funds are cut. "It depends on who you talk to," said Bikel. "Programs [in general] would suffer the most. You can't reduce that much in administrative costs -- the funds for staff and review panels. You will still have the same grant-application load. You still need people to read them. In programs, the marginal things will go under. The Los Angeles Symphony won't go broke. It's the small theaters, the small dance companies that depend on that federal dollar. A lot of those might simply disappear. They can't go and ask elsewhere for money unless they have that federal dollar. That federal dollar is a federal good housekeeping seal of approval for them."