The most dangerous piece of foreign policy legislation in memory is moving through Congress. It is titled the Taiwan Security Enhancement Act. Introduced in the Senate by Jesse Helms (R-N.C.) and Robert Torricelli (D-N.J.) and in the House by Rep. Tom DeLay, it ought to be called the Taiwan Security Reduction Act.

If enacted, the legislation would diminish Taiwan's security, alarm friends and allies and greatly increase the chances of armed conflict between America and China. Though a watered-down version of the act probably soon will pass the House, its future is less certain in the Senate. If the bill passes the Senate too, the president must (and I believe will) veto it. If the president rolls over, the American people can expect renewed military tension in the Taiwan Strait. Taiwan citizens will risk falling markets, fleeing capital and diminished security. Washington can forget Sino-American cooperation on major issues. And East Asia can prepare for an arms race.

That the bill is moving through the legislative process now is due, in part, to a Taiwan lobbying effort fueled by money given to politicians, Washington think tanks and law and public relations firms though a variety of probably legal channels. This river of money makes the alleged illegal campaign contributions from the People's Republic of China (PRC) look like chump change.

Beyond money, the legislation draws life from a genuine problem. Over the past two decades, Washington has negotiated complex arrangements with Beijing governing ties with the PRC and Taiwan. These agreements, along with the 1979 Taiwan Relations Act, create ambiguities about the circumstances under which Washington would respond to violence in the Taiwan Strait.

Generally, such ambiguity is undesirable, because uncertainty can foster dangerous miscalculations. In this case, however, if Washington makes it clear that it will support Taipei militarily no matter what it does, we inadvertently encourage unnecessarily provocative behavior by Taiwan that might draw the United States into bloody conflict with Beijing. Conversely, were Washington too timid in deterring Beijing from resorting to force in the Taiwan Strait, we could jeopardize Taiwan's security by leading the PRC to think we might not respond. Some ambiguity is the most prudent middle course.

And in fact there isn't that much ambiguity in the current situation, if Taiwan is not provocative. After all, President Clinton sent a 16-ship flotilla, with two aircraft carriers, to the waters off Taiwan in 1996 when Beijing fired missiles in the island's direction. Further, from 1994 to 1996 Taipei was the fourth largest recipient of U.S. arms, with the pace of deliveries jumping dramatically in 1997, largely because of the sale of F-16s. Taiwan buys so much weaponry that it has had difficulty absorbing, maintaining and operating the equipment. In 1997 the United States sold 8.5 times the value of weapons to Taiwan (in constant dollars) that it did in 1981.

The U.S. legislation is doubly provocative because it both inflames Chinese nationalism and strengthens the PRC military's claims for more weapons and money, on the one hand, and reinforces those in Taiwan who think they can manipulate the U.S. political system at will, on the other. Further, the act perpetuates the myth in Taiwan's presidential palace that with 1.3 billion people only 95 miles away, Taiwan's security can be principally secured by military means.

If this legislation emerges from Congress in anything like the Senate version, it deserves a presidential veto for five reasons:

* It is unnecessary. The president already has the legal authority to sell Taiwan the weapons it needs.

* The framework by which U.S.-PRC-Taiwan relations have been managed for the past 20 years has permitted Taiwan to make dramatic economic and political progress. If the current framework "ain't broke, don't fix it."

* The legislation directs the executive branch to establish direct military communications, training and other relationships with the Taiwan armed forces, which amounts to a functional reestablishment of the 1955 Mutual Defense Treaty with the island. This would run counter to the terms on which diplomatic relations between Washington and Beijing were established in 1979.

* The bill encourages the transfer of upper-tier antimissile defenses to Taiwan that do not yet exist and which the United States may eventually wish to keep under its own control. Such a declaration would fuel a PRC missile buildup now, while an easily overpowered defense for Taiwan is years away.

* The U.S.-China relationship has enough problems now without saddling this administration with still more--or a new administration with a deteriorating security environment.

The writer is director of China studies at the Nitze School of Advanced International Studies of Johns Hopkins University and the Nixon Center in Washington.