The real threat to President Bush's decision to extend most-favored-nation trade status to Communist China another year may be Iran, Uncle Sam's old nemesis, whose purchasing agents are believed to be in Beijing right now buying nuclear weapons technology.

American diplomats refuse to talk about this as Congress nears a showdown on Bush's demand to extend MFN without conditions. But with Undersecretary of State Reginald Bartholomew just back from Beijing, where he pushed hard to enforce nuclear nonproliferation by cash-hungry China, the administration is getting worried. If Beijing is caught selling nuclear technology to Tehran, MFN for China will be DOA -- dead on arrival.

"This is the president's real Achilles' heel on MFN for China," Sen. John McCain (R-Ariz.), one of the Senate's better informed Asian specialists, told us. If true, that suggests the bitter congressional battle over China's MFN status will shift from human rights, where Bush has been taking a big but sustainable hit, to nuclear weapons, an issue that may cut deeper.

A special $200 million fund devoted to Iran's neophyte nuclear weapons program was approved in Tehran for the year that ended March 31. The current year's plan is believed to be even more costly in a development sequence sealed off from the Islamic nation's publicly displayed Atomic Energy Organization. The weapons agenda is centered in Qazvin, near the Caspian Sea, and is described by the few scientists and government and military officials aware of it as "the secret plan."

Most troubling to U.S. intelligence is the fact that the infamous Revolutionary Guards are believed to be in charge of the program. They launched the fanatical Islamic fundamentalist movement headed by the late Ayatollah Khomeini that toppled the shah and made hostages of the American Embassy staff in 1979. They were the spear point of the futile eight-year war against Iraq's Saddam Hussein.

At a private Capitol Hill luncheon Monday given by Rep. Mervyn Dymally (D-Calif.) and other members of Congress for Mohammed Mohaddessin, the foreign policy spokesman for Iran's leading anti-Islamic movement, the People's Mujahedin, Mohaddessin said Iran is determined to develop nuclear weapons on a cash basis -- and has the money to do it.

He did not mention the Beijing connection, but later he spelled out to several members of Congress that Tehran views China as one of three prime possibilities to sustain its nuclear weapons-development program (along with Pakistan and Argentina). Iranian President Hashemi Rafsanjani, who is regarded by some administration officials here as a "moderate" post-Khomeini leader, is believed to be particularly eager to focus on China as the supply point for Iran's program, because China is in such desperate need of hard currency. Iran's cash comes from the sale of 2.6 million barrels of oil daily.

While here, Mohaddessin said privately that Iran signed an agreement last December for a $500 million nuclear deal with Argentina. Rafsanjani, he said, also personally arranged to send about 20 Iranian nuclear technicians to Beijing for special training. Other Tehran approaches have been made, with some success, to western European nations, including Italy, Germany and France.

But these countries are not about to be deprived by the U.S. Congress of their most-favored-nation status. What has made China so vulnerable is its bloody human rights record. After the June 1989 Tiananmen Square slaughter of students, workers and political activists, Bush froze military relations between the two countries and sent key aides on two trips to Beijing, one of them secret. They warned China to improve its human rights performance or risk a setback in U.S.-China trade. It did not work, but Bush refused to take the next step and end MFN or to let Congress do so. He won.

But this year may be different. With Bush having smashed Iraq for its seizure and occupation of Kuwait, many politicians of both parties wonder why he insists on favored treatment for China, which seized and still occupies Tibet, brutalized Tiananmen Square and continues political trials. "This encourages the Chinese hard-liners," famed dissident scholar Fang Lizhi told The Washington Post. "They will feel no pressure."

Pressure now may be forthcoming. The help China is giving Iran's nuclear ambitions may be the silver bullet that kills the president's plan to let his old Beijing buddies have it all their way.