Defying a presidential veto threat, 19 Republicans joined all 50 Democrats and one independent in voting to limit debate on a truck inspection measure that has generated accusations of discrimination from the White House, threats of retaliation from Mexico, and frayed tempers in the usually decorous Senate.

Although Thursdays vote was on a procedural motion to end a filibuster, it was considered a clear signal of Senate intent. A final vote on the truck inspection proposal is expected within a few days, unless a compromise is reached in the meantime.

President Bush, speaking before the Senate vote, expressed concern about efforts to block implementation of a key provision of the North American Free Trade Agreement. It is wrong for the Congress to discriminate against Mexican trucks, he said.

The 70-30 vote, which followed even stronger action by the House last month, underscored the intensity of opposition to the administrations plan to let Mexican freight haulers operate throughout the United States beginning next January.

Currently, under a restriction put in place by the Clinton administration, Mexican trucks are confined to a roughly 20-mile zone along the border. In February, a NAFTA arbitration panel ruled that the restriction violated the terms of the treaty.

Although the Bush administration has proposed its own plan for beefing up border inspections, many members in Congress say it is too lenient. The Senate is considering a tougher inspection program proposed by Sens. Patty Murray, D-Wash., and Richard Shelby, R-Ala. It would require Mexican trucks to undergo a series of safety checks before they would be allowed to travel beyond the border zone. Among other things, U.S. inspectors would inspect trucks and audit records at trucking company sites in Mexico, and verify drivers licenses and insurance coverage at border crossings.

Administration officials say the additional safety checks are unnecessary and could delay implementation of the NAFTA requirement by as much as two years. They say Mexican trucks and drivers operating in the United States would be subject to U.S. safety rules under the administrations less rigid proposal.

But Murray contended that under the administrations plan, all that Mexican truck companies will need to do is check a box saying that they have complied with U.S. regulations, and their trucks will start rolling across the border.

The administration has proposed requiring trucking companies to certify they will meet U.S. safety rules. Companies that pass muster will receive a conditional permit for 18 months, during which their records would be audited and their trucks inspected.

The administrations Senate allies proposed to establish along the southern border a truck inspection program patterned after Californias program, which has been credited with reducing the number of Mexican trucks taken out of service for safety violations.

Administration officials say the additional measures called for in the Senate measure could deny Mexican trucks full access to U.S. roads for two years or longer. That, the administration has warned, would violate NAFTA and make the United States subject to more than $1 billion a year in sanctions.

The White House has threatened to veto any legislation that prevents the United States from fulfilling its NAFTA obligations to open the U.S. borders to Mexican motor carriers that can satisfy U.S. safety and operating standards.

Even if the Senate gives final approval, the inspection provision, which is included in a $60 billion transportation funding bill, must go to a House-Senate conference committee, giving the administration time to try to work out a compromise. Last month, the House voted to prohibit U.S. officials from processing applications from Mexican trucking companies to travel throughout the United States unless stronger measures are taken to ensure their safe operations.

There were about 4.5 million border crossings in fiscal 2000, according to federal officials. Of all Mexican trucks inspected, 37 percent were taken out of service because of safety violations such as defective brakes and bad tires; the rate was more than 50 percent at some crossings in Arizona and Texas. In California, 27 percent of Mexican trucks inspected were ordered off the road, comparable to the out-of-service rate for U.S. rigs.