President Carter yesterday signed an order approving the sale of 38 tons of enriched uranium to India, forcing a showdown with Congress about U.S. policy on the spread of nuclear weapons.

In signing the order, Carter overruled a unanimous vote by the Nuclear Regulatory Commission against the shipment of the nuclear fuel to India. The order now moves to Congress, where the House and Senate have 60 days to adopt a joint resolution of disapproval to prohibit the sale.

"I will introduce a resolution of disapproval that will pass the full House," said Rep. Jonathan B. Bingham (D-N.Y.), chairman of the House subcommittee on international economic policy. "I don't know a single member of the House Foreign Affairs Committee [parent of the subcommittee ] who will not sign it," he said.

Carter's approval of the unranium sale to India is a bending of his own policy on the spread of nuclear weapons. Carter is the first president to insist he see every application for the export of uranium.

"This is what bothers me the most," Bingham said. "We are going to have a tough time explaining to our allies why we caved in on this one export."

While there is a good chance the House will disapprove the sale to India, there is at best only an even chance the Senate will do so.

The Carter administration has lobbied hard on the Senate side for the sale, sending Deputy Secretary of State Warren Christopher twice in the last week to testify in its behalf before joint meetings of the Senate Foreign Relations and Governmental Affairs committees.

Neither of the two committees' leaders have said they will introduce resolutions of disapproval on the Senate side. Senate Majority Whip Alan Cranston (D-Calif.) is one of the few high-ranking Senate members to express his active opposition to the sale.

"I oppose this export because I believe it would harm our effort to discourage Pakistan from its pursuit of a nuclear weapons capability," Cranston told the Foreign Relations Committee Wednesday. "I believe this export to India would do grave injury to the nuclear nonproliferation and foreign policy interests of the U.S." In signing the approval order, Carter told congress that rejection of the sale would do more harm than good in U.S. relations with India and U.S. policy on the spread of nuclear weapons.

"This action reflects my judgement that nonproliferation would be set back by withholding these exports," Carter said in a message to Congress, "and that our failure to supply this fuel could seriously jeopardize other important U.S. interests."

Carter said he is just as concerned as the Congress in that India had refused to sign the Nonproliferation Treaty prohibiting the spread of nuclear weapons and refuses to open up its nuclear facilities to international inspection.

"India's failure to acccept international safeguards and its failure to commit itself not to conduct further nuclear explosions are of serious concern to me," Carter said. "These exports will help us to maintain a dialogue with India in which we try to narrow our differences on these issues."

India detonated a nuclear device in 1974. Deputy Secretary of State Christopher told the Senate Foreign Relations Committee that the State Department has no evidence that India plans a second test explosion. If India conducts a second nuclear test, Christopher said, the United States would stop all nuclear shipments.

The key Senate members in the issue are Sen. John Glenn (D-Ohio) and Sen. Charles H. Percy (R.-Ill.), members of both the Foreign Relations and the Governmental Affairs committees that have jurisdiction over nuclear exports.

Glenn would like the United States to delay the shipment until August when the countries signing the Nonproliferation Treaty meet in Geneva to review it, but is sympathetic to the Carter administration's concern over maintaining good relations with India.

"My tendency is to be realistic," Percy said in an interview. "My feeling is if we refuse to supply the fuel, that will release India from the stipulations of the original contract."

The original contract, signed in 1963, calls for the United States to ship enriched uranium for India's nuclear power station at Tarapur, north of Bombay, for 30 years. The contract also states that India cannot reprocess any of the spent fuel to extract its plutonium, used to make nuclear explosives. An estimated 2,200 pounds of plutonium in the fuel already has spent at the Tarapur station enough for 100 atomic explosives.