Sen. Deniel Patrick Moynihan (D-N.Y.). chairman of the Senate Public Assistance Subcommittee, who had been expected to lead the battle for President Carter's welfare program. said yesterday that the legislation is "grievously disappointing" and the proposed benefits "not high enough."

Moyniban, author of a similar plan as an official of the Nixon administration, told a House welfare subcommittee that the administration proposal for overhaul of the entire welfare system had been written in such a way that it failed to attact "the necessary coalition of . . liberal-minded individuals and groups" needed to push the plan through Congress.

Moynihan's statement indicated further grave perits for the already-embattled proposal.

Moynihan said his conversations with officials of the AFL-CIO, the League of Women Voters. the Black Caucus, various state and country welfare officials and the Leadership Conference on Civil Rights in recent weeks had convinced him that they are seriously unhappy with benefit levels and with lack of guarantees that states will undermine the plan when it comes down to administrative detail.

"I didn't realize the extend of hostility to it." Moynihan said in an interviews.

Unless changes are made and the administration puts together a stronger coalition by satisfying some of the groups he names. Moynihan said. "Then we must entertain the grave possibility that welfare reform can never come about." He added. "We will have to rewrite it." He said he still favors the basic principles, but that it must be reworked.

Moynihan said he had applauded the President's welfare message and had agreed to sponsor the bill two weeks ago, but said he had first made a careful analysis of the actual legislative provisions in preparation for a debate last weekend with conservative writer William Buckley. His staff also analyzed it, he said, and, "The more we read it, the more we said. 'My God. it's certainly not liberal legislation.'"

Moynihan conceded that he is extremely angry and disappointed at administration efforts to block a Moynihan bill giving the states $1 billion in special "fiscal relef" on welfare costs over the next 13 months - well in advance of the effective date of the new plan.

But he said attempts by the Carter administration to block that money - New York would get $141 million - are not the reason for his statement on the big welfare plan.

He said overall the presidential plan goes in the right direction, but "not far enough."

For example, he said, a family of four without other income would be given assistance ($4,200 a year) "that is less than two-thirds of the present poverty line."

He also said all but about a dozen states would have to supplement federal welfare minimums to maintain benefit levels as high as paid under the existing welfare system, which most people consider a poor one.

He said proposed penalities for the welfare family whose head refuses to work are too severe, and he also criticized both the structure and the pay level of the public service jobs program that forms part of the welfare proposals. He said the pay levels would be too low.

In Dallas. Secretary of Health, Education and Welfare Joseph A. Califano Jr. said. "I am sorry to hear that Sen. Moynihan is now grievously disappointed with a program that he termed "magnificently drafted" less than two months ago."