A conflict that could strain relations for months between the United States and India comes to a head today when the key provision of a law Congress passed in 1978 goes into effect.

It is the Nuclear Nonproliferation Treaty, which set today as the deadline for countries that want to buy enriched uranium fuel from the United States to do one of two things: sign the treaty prohibiting the spread of nuclear weapons or open up all atomic plants to outside inspection to make sure plutonium from spent nuclear fuel is not being diverted to nuclear weapons use.

India has done neither. The State Department has engaged in on-and-off negotiations the past two years with three Indian governments and has come away empty-handed.

"All we've really asked for from the present Gandhi government is some kind of assurance the plutonium in the spent fuel will not be used for explosives," one State Department source said. "We have yet to get that assurance."

At stake now is the export of 21 tons of uranium fuels from the United States to refuel nuclear power plants at Tarapur, India. If the five commissioners of the Nuclear Regulatory Commission were to vote today, they might vote against it.

"I would be very hard put to vote for it today," one of the five commissioners confided. "I just don't see how we can vote for a license without any formal assurances from India that they are willing to conform to the provisions of the nonproliferation act."

Time is running out on the Indian export application, which came to the NRC in October 1978. The eight tons of uranium the NRC approved for shipment to India last year has been made into fuel rods at the Indian fabrication plant at Hyderabad, which was counting on getting the next shipment this month to keep the plant open.

If India does not the uranium in the next few months it will have to start reducing the power output of its four reactors at Tarapur to conserve uranium. Sources at the State Department say that if India reduces. Tarapur's power output to 70 percent, it can keep the plants running until June 1982. If the plants keep running full tilt, they will have to shut down sometime next year.

The Tarapur export license is pending at the State Department which may keep it until it hears from the Gandhi government about its spent-fuel intentions. One source at State said the Gandhi government has only to give assurances that the plutonium in the Tarapur fuel would never be used to make explosives.

If the State Department were given assurances, the export license would go to the NRC.

If the NRC voted against the shipment, the license application would go automatically to the White House where President Carter likely would approve it. Should that happen, Congress would have 60 days to override his decision or stay silent and let the uranium go to India.

Key Senate and House leaders have mixed emotions about overruling the White House. Rep. Jonathan B. Blingham (D.-N.Y.), chairman of the Foreign Affairs subcommittee on international economic policy, has said he does not want to see the fuel shipped without assurances it will not be used for weapons but might let it go if India gave assurances.

Sen. John G. Glenn, Jr. (D-Ohio), chairman of the Senate Governmental Affairs subcommittee on energy, nuclear proliferation and federal services, has not made up his mind. Said one aide: "Sen. Glenn believes that if the United States waives the law in this case, it's the ultimate step backward in proliferation matters. On the other hand, he understands the politics of the whole matter."