Latin nations worried that trade legislation before Congress could hurt their economies found an ally yesterday in the Reagan administration.

A special committee of the Organization of American States reached almost complete agreement with administration trade officials on parts of the trade bill that they want eliminated, reported Mario Rodriguez Montero of Mexico.

Rodriguez, minister for trade affairs in the Mexican Embassy here, spoke as the head of the special OAS panel that yesterday ended two days of meetings on the impact of U.S. trade legislation on the economies of Latin nations.

Instead of arguing with the administration over the trade bill, the Latin OAS members found wide areas of agreement, Rodriguez said. On the advice of administration officials, the Latin nations decided to take their case to Capitol Hill, where the trade debate is due to begin Tuesday or Wednesday.

"We are not going to wait. There is an urgency to do something as soon as possible," Rodriguez said.

At the White House, meanwhile, President Reagan met with the Senate Republican leadership and warned for the second time this week that he will be forced to veto trade legislation if the Senate doesn't improve the bill that passed the House in April. He asked the Senate leadership to eliminate the most objectional parts of the House bill.

Two of the Republican leaders, Sens. Robert Packwood (Ore.) and John C. Danforth (Mo.), said there is still a chance that Congress can pass a bill the president will sign.

"The president has asked us to hold firm, to fend off amendments on the Senate floor, and I told him I kind of feel like a commander in the center of the fort; you never know where the attack is coming from," said Packwood.

"The bill could get worse and worse or it could get better and better," he added.

More than 100 amendments have been proposed for the bill in the Senate, where the debate is expected to last for two to three weeks.

Senate Majority Leader Robert C. Byrd (D-W.Va.), announcing plans to bring the trade bill to the floor next week, warned his Senate colleagues yesterday that the debate could include day, night and Saturday votes. He indicated that he would keep the Senate working on the stalled campaign financing bill as well as trying to complete trade legislation.

The appeal to the White House by the special OAS committee reflects the anxiety being felt around the world as the U.S. Congress tries to reverse five years of record trade deficits. The legislation is meant to increase American overseas sales, but trading partners fear it will cut their imports to the United States.

In Singapore, Foreign Minister Suppiah Dhanabalan told Secretary of State George P. Shultz at the meeting of the Association of Southeast Asian Nations (ASEAN) yesterday that Southeast Asia is a victim of unabated protectionist pressures from the United States. "ASEAN can only watch in apprehension," United Press International quoted Dhanabalan as saying.

Shultz, though, warned those fast-growing Pacific Rim nations to diversify their markets because they cannot depend on the United States to buy more of their products.

Rodriguez said Mexico's concerns include a provision to reduce U.S. dependence on oil imports that could result in an oil tax; new restrictions on steel imports; a new definition of subsidies aimed at natural resources such as oil and gas that could hurt Mexico's cement, ammonia and carbon black industries, and the removal of the president's discretion to impose trade restrictions.