President Reagan's recent diplomatic approach to the conflict in Central America has opened an unusual rift with the 1988 Republican presidential candidates, who say they want military aid to the Nicaraguan rebels to continue despite the talk of peace negotiations.

In his recent proposal with House Speaker Jim Wright (D-Tex.), the president agreed to postpone his formal request for renewed military aid while diplomatic moves are under way, but most of the Republican candidates have called for moving ahead with contra aid regardless of the diplomatic maneuvering.

Rep. Jack Kemp (R-N.Y.) announced yesterday that he would introduce legislation when Congress returns next month to provide $310 million in military aid to the contras for the next 18 months and ask for a vote before the existing $100 million in aid expires Sept. 30.

Former Delaware governor Pierre S. (Pete) du Pont IV has called for terminating diplomatic relations with Nicaragua and making a long-term financial commitment to the rebels.

Even Vice President Bush, long a loyal administration soldier on foreign policy issues, has criticized the Guatemala City peace plan signed by the five Central American presidents, which initially received qualified support from Reagan. Bush vowed not to leave the contras "twisting in the wind, wondering if they are going to be done in by a peace plan." The vice president also expressed doubt whether Nicaraguan president Daniel Ortega, who signed the agreement, is committed to democratic reforms. "Regrettably Ortega is what he says he is," Bush told Miami radio station WINZ. "He's a Marxist. And I don't know of one who is committed to democracy."

Republican strategists said military aid to the contras has become what one called a "weather vane" issue among GOP primary voters. Although the American public overall remains sharply divided about sending military aid to the contras, key voter groups in the Republican primary contests are strongly in favor of continued aid. For example, pollster Richard Wirthlin said his recent surveys show that people who say they will vote in the 1988 Republican primaries favor contra aid by 68 to 29 percent.

A Washington Post-ABC News survey this month found that Republicans favor continued aid to the contras by 50 to 45 percent, while Democrats overwhelmingly opposed it, 71 to 24 percent. Most of the 1988 Democratic presidential candidates have criticized Reagan's policies in Central America and called for a diplomatic resolution rather than more aid.

"It's a democracy issue, a freedom issue, and a tough-on-defense issue," said David Carmen, a GOP political consultant and spokesman for the presidential campaign of former senator Paul Laxalt (R-Nev.), who also backs continued contra funding.

Another leading GOP strategist put it this way: "It's one of those issues where if you're right, you're very right, but if you're wrong, you're absolutely ghastly wrong. If over the next year or two years, if Nicaragua should become a base to subvert El Salvador, Guatemala, Coasta Rica, then the conservatives tend to have long memories . . . . The risk of being on the wrong side of this issue in 1988 and 1989 is virtually unacceptable."

The recent diplomatic initiatives, including the Reagan-Wright plan and the Guatemala City accord, have left some longtime Reagan backers questioning how the president came to endorse either. The Reagan-Wright proposal has been criticized for lack of guarantees for the contras; the Guatemala City agreement for lack of an enforcement mechanism to lock in democratic reforms in Nicaragua.

Kemp yesterday blamed White House aides for giving Reagan "bad advice." Conservative activist Paul M. Weyrich issued a statement saying Reagan had been misled. "If Mr. Reagan really believes he is not killing the contras with his new peace plan" and his partial embrace of the plan by Costa Rican president Oscar Arias signed in Guatemala City, "then someone is not telling him the truth," Weyrich said.

Reagan has sent conflicting signals about the Guatemala City accord, at first welcoming it but more recently raising questions about it. The White House also has wavered on Reagan's plans for seeking renewed aid to the contras.

But the Republican candidates unanimously support continued aid.

Senate Minority Leader Robert J. Dole (R-Kan.) called last week for reauthorization of aid to the contras now, with humanitarian aid to be provided if a cease-fire is arranged, and military aid if the Sandinistas block or stall an agreement.

Another GOP candidate, former secretary of state Alexander M. Haig Jr., praised the bipartisan Wright-Reagan effort but criticized the Guatemala City accord for omitting "the Cuban and Soviet dimensions of the problem," according to spokesman Dan Mariaschin.

Television evangelist Marion G. (Pat) Robertson also supports renewed contra aid and would support whatever request Reagan made, spokesman Scott Hatch said.