I don't know what's worse -- that the Teamsters and other protectionist groups have waged a largely successful campaign to keep Mexican trucks out of the United States or that Congress is buying it.

But one thing's for sure: If the transportation bill that will soon land on President Bush's desk doesn't repeal the current restrictions on Mexican trucks -- ones that go beyond those imposed on trucks from Canada, our other North American Free Trade Agreement partner -- the president should veto it.

To hear the foes of free trade tell it, letting Mexican truckers move freely on U.S. highways -- for now, they're limited to a 20-mile zone north of the Rio Grande -- would a) spark countless accidents, b) make our air unbreathable, c) open the floodgates of drug trafficking and d) exploit the very truckers who are wreaking such havoc. Nonsense.

Keeping the restrictions in place is blatantly unfair, for one thing. When other nations negotiate trade agreements with the United States, they expect the terms of those agreements to be honored. Yet when we agreed to open our borders to Mexico, we unilaterally suspended the section of NAFTA that deals with trucking.

Mexican President Vicente Fox has demanded that this be addressed. If not, he's promised to stop American trucks from entering Mexico.

Who can blame him? The reason Canadian trucks can roam the entire expanse of the United States and Mexican trucks can't has little to do with concerns about safety, drugs or the environment. It has much to do with pressure from protectionist groups, the fact that 1.4 million Teamsters live in Canada but none live in Mexico, and the reality that Congress is driven less by principle than by sensitivity to the political winds.

Mexican trucks that operate in the 20-mile commercial zone are sidelined for safety problems only slightly more often than American trucks are. Inspections at two ultra-modern stations on the California border found 27 percent of Mexican trucks fell short of U.S. safety standards, a slight decrease from last year. But they also found that 24 percent of American trucks fell short.

Protectionists claim that keeping Mexican trucks from operating throughout the United States helps prevent exploitation of Mexican truck drivers, who earn, according to the Teamsters, as little as $7 per day. This criticism would carry more weight if it didn't come from a union that allows its own workers to be exempt from minimum-wage laws and to be paid by the mile, which forces many truckers to drive far longer each day than permitted by law. (That's why if an American trucker swears on his log book that something is true, he's putting you on.)

Should Mexican trucks be required to meet the same safety standards as U.S. and Canadian trucks? Of course. No one wants 18-wheel rigs carrying 88,000 pounds of cargo careening down America's highways recklessly. Should Mexican drivers be subject to the same scrutiny in terms of training and record-keeping of accidents and alcohol and drug violations as American and Canadian truckers? Of course.

Mexican trucks and drivers may be behind the United States and Canada in record-keeping and equipment quality, but they're catching up fast in both areas and would catch up faster if America ended its prohibition on selling used trucks in Mexico.

The Mexicans will continue to upgrade. They want badly to capture a piece of our gargantuan market and know quality concerns could hurt them competitively. As they improve, we should undertake steps to help bring them up to standards -- not violate international treaties, snub a neighbor and No.2 trading partner, and let self-interested union leaders determine U.S. trade policy.

U.S. immigration officials have asked that 80 more federal truck inspectors work along the borders of Texas, New Mexico, Arizona and California, and their request should be granted. Truckers who count on being inspected commit far fewer violations than others.

NAFTA is working, and Sen. Patty Murray, D-Wash., who amended the transportation bill to include the extra sanctions on Mexican truckers, knows it. It's creating high-paying jobs, generating commerce and bringing neighbors together. That's why we can't turn back -- and why we can't allow unfair exceptions to NAFTA's rules.

And why, if Sen. Murray's amendment is part of the transportation bill, President Bush must veto it.

Edwin Feulner is president of The Heritage Foundation ( www.heritage.org), a Washington-based public policy research institute