The newly proposed nuclear-cooperation agreement between the United States and China poses unprecedented issues for Congress, torn between wariness of China's communist regime and the desire for a friendly trading relationship with the world's most populous nation.

Critics of the pact are an unlikely combination of liberals worried that it would permit further spread of nuclear weapons and conservatives suspicious of anything China says. Administration defenders of the pact, confident that they have met everyone's objections, regard the agreement as a landmark both in U.S.-China relations and in controlling future growth of the international "nuclear club."

In a letter submitting the pact to Congress, President Reagan said it would have "a significant, positive impact on overall U.S.-China relations" and would provide "substantial economic benefit" to the ailing U.S. nuclear power industry.

"Based on our talks with the Chinese," the letter said, "we can expect that China's policy of not assisting a nonnuclear-weapon state to acquire nuclear explosives will be implemented in a manner consistent with the basic nonproliferation practices com-mon to the United States and other suppliers."

Critics are concerned because China's commitment is purely verbal. They fear the agreement would set a precedent for more handshake deals on nuclear matters, and worry that what is written down either has too many loopholes or is just the administration's description of what China supposedly thinks.

The pact is identical to the one Reagan brought back from Peking in April 1984. Pushed hard by the nuclear industry, the agreement sets out conditions under which the industry may bid for a share of China's nuclear power future, which could involve $6 billion or more in foreign contracts.

But the pact was not submitted to Congress until last Wednesday, partly because Reagan's own arms experts were worried about the language and partly because of intelligence reports that Chinese technicians were at work at Kahuta, Pakistan, the alleged site of a secret nuclear weapons development facility.

The administration now says the Chinese have left Kahuta, if they were ever there, and that China has both declared its commitment to nonproliferation principles and demonstrated it by joining the International Atomic Energy Agency, a United Nations affiliation of more than 100 nations committed to audits, inspections and listings of their nuclear activity collectively known as "safeguards" against military use.

Richard T. Kennedy, Reagan's special ambassador on nonproliferation policy, made the rounds of Capitol Hill last week to relate details of his June talks in Peking and to give assurances of China's good intentions. A classified summary of those talks, which Kennedy wrote but the Chinese are said to have approved, is expected to be part of the administration pitch to defuse objections to the pact.

But China has not signed the Nuclear Nonproliferation Treaty that 121 other nations have signed, and it has so far refused to permit inspections of its nuclear facilities, although it says it will impose safeguards on its own nuclear sales. It is offering to store spent nuclear fuel from the world's nuclear power reactors at a yet-to-be-built site, thought to be in the Gobi Desert, that it says could hold 5,000 tons by the year 2000.

In a March 1984 letter expressing concern about that plan, Sen. Dan Quayle (R-Ind.) and six other senators noted that "50 tons of weapons-usable plutonium (6,000 weapons' worth) could be extracted" from that spent fuel by reprocessing.

Asked at a House subcommittee hearing last week whether the pact allows a U.S. veto, Kennedy said flatly, "Yes." But some critics think the language is "suggests a bias" in favor of approving reprocessing requests, said an aide to Sen. John Glenn (D-Ohio), ranking Democrat on the nonproliferation subcommittee.

The pact also says it will take precedence over domestic laws as a treaty would. That, the aide said, is "troublesome language" that might preclude any future tightening of U.S. law regarding nuclear trade with China.

Glenn said he has wants answers to questions about "a number of hedges, ambiguities and gaps" before deciding whether to back the agreement.

There are also unresolved reports that China secretly sold low enriched uranium to South Africa and heavy water, used in weapons development, to Argentina and India. A large research reactor reportedly under construction in Yongbyon, North Korea, may be getting some kind of help from China.

"It's critical we get on the record the past history of China's heavy water and uranium exports, as well as their alleged activities in Pakistan," said Leonard S. Spector, former chief counsel of the Senate subcommittee on nuclear proliferation and now a senior associate at the Carnegie Endowment for International Peace.

Sen. William Proxmire (D-Wis.), a leading critic of the agreement, said he was "very anxious" to study the full package and still had several questions remaining after Kennedy's briefing. "I'm not convinced just on the basis of a conversation with anybody," Proxmire said.

Senate Foreign Relations Committee Chairman Richard Lugar (R-Ind.), said yesterday he has "no objections" to the pact, and added that no senator has so far asked him to hold hearings on it. The House Foreign Affairs Committee rescheduled yesterday's hearing to next Wednesday.