A bipartisan group of 106 House members yesterday urged President Reagan to "respond positively" to a joint Mexican-Venezuelan proposal for negotiations to relieve tensions in Central America.

But U.S. officials said privately that, while the administration regards the Mexican-Venezuelan initiative as "a serious proposal" deserving study, the United States continues to be wary of any plan that might force it into formal talks with Nicaragua's leftist Sandinista government.

The initiative was put forward by presidents Jose Lopez Portillo of Mexico and Luis Herrera Campins of Venezuela in a letter to Reagan on Sept. 7. It stressed resolving border tensions between Nicaragua and Honduras, and it suggested that current low-level contacts between Washington and Nicaragua be upgraded to "facilitate a genuine negotiation able to overcome the difficulties."

The proposal, and the House members' letter supporting it, came at a potentially awkward time for Reagan. He will meet at the Mexico-California border on Friday with Lopez Portillo's elected successor, Miguel de la Madrid Hurtado, and the administration is anxious to make a good start in relations with the new Mexican government. For one thing, the United States is concerned about Mexico's severe financial crisis; it also hopes to divert that country from its past sympathies toward Nicaragua and leftist insurgent movements in Central America.

In addition, the administration wants to guard against further criticism in Congress that its Central America policy is frozen in an uncompromisingly anti-communist mold. Such criticism could rekindle the controversies of recent months about how genuinely the administration is supporting political and social reform within the region and create new opposition in Congress to U.S. military aid for El Salvador and Honduras.

The president's response to all this, spelled out in a letter sent last weekend to the Mexicans and Venezuelans, was a warm but noncommittal reply to their proposal. Although Reagan's letter has not been made public, a copy has been obtained by The Washington Post.

In it, Reagan said, "I believe that any meaningful attempt to address the problems of Central America must be within a regional con-text. . . ." That, he continued, should achieve "democratic pluralism within each nation" of the region, "an end to support for terrorist and insurgent groups in other countries of the region," the banning of heavy weapons imports into Central American countries and limits on foreign military advisers in the region.

These are the conditions that Washington has been trying for several months to press on Nicaragua, which the administration contends is being transformed by its Sandinista leaders into a Cuban-style military state and base of support for leftist insurgents in El Salvador and other countries of the region.

Reagan's emphasis on a "regional context" referred to the approach that the administration began taking toward Central America shortly after George P. Shultz replaced Alexander M. Haig Jr. as secretary of state this summer.

At its core, it represents no change from the administration's determination to turn back what it sees as a communist-led drive to win control of the region. But, in contrast to Haig's confrontational tactic of talking constantly about the situation in threatening terms, the administration has shifted to a less visible strategy of encouraging the region's democratic governments to carry the public burden of isolating Nicaragua. That approach paid off in what the administration regards as a major success at a meeting earlier this week in San Jose, Costa Rica. There, Belize, Colombia, El Salvador, Honduras, Jamaica and Costa Rica jointly endorsed a broad framework for peace and democracy that included the same points enumerated in Reagan's letter.

Mexico and Venezuela, the two most powerful countries neighboring Central America, were not represented at the meeting, apparently because they advocate a more conciliatory policy toward Nicaragua. However, U.S. officials denied yesterday that they were excluded from the San Jose meeting because of a desire to present a solid front of countries that basically share Washington's views.

The officials noted that some of the points stated by Reagan and the San Jose meeting also are in the Mexican-Venezuelan proposal, and they said that further exploration could bring the two approaches into agreement. From the U.S. point of view, the officials added, the main problem with the Mexican-Venezuelan initiative is that it concentrates on the Nicaragua-Honduras border troubles while the United States wants a broader, "regional context" approach that will isolate Nicaragua or force it to modify its behavior.

The officials reiterated U.S. denials that such a change on Nicaragua's part is a condition for formal negotiations between Washington and Nicaragua. But they also made clear that, given Washington's conviction that Nicaragua is still intent on aggression toward its neighbors, the administration does not believe there would be any purpose to negotiating now.