Last April, Rep. Edwin V.W. Zschau of California was one of 40 House Republicans to oppose President Reagan's request for military aid to the rebels fighting the Sandinista government of Nicaragua. Today, Zschau is scheduled to be the leadoff speaker for the Republicans in supporting Reagan's new aid request.

In a statement yesterday, the former Silicon Valley educator and entrepreneur said he despaired of the diplomatic option with Nicaragua and had decided that "we will wait forever" for serious negotiations to begin "unless stronger action is taken."

Unmentioned in his statement was the fact that Zschau is now one of 10 contestants in the June 3 Republican primary for the nomination against Sen. Alan Cranston (D-Calif.). The other House member in the race, Rep. Bobbi Fiedler, backed the president's request last April and on other occasions. All others who are considered major contenders for the nomination support the administration's policy of aiding the counterrevolutionaries, or contras.

Zschau, who was campaigning in California yesterday, was not available for comment, but a prominent supporter who said he had discussed the issue with the congressman said that "he had no choice."

"Ed is a moderate Republican and a northern Californian," he said, "and that makes him doubly suspect to the conservatives in the south. Some of us think he's our best bet to beat Cranston, but he'd never get through the primary if he were one of the handful of Republicans who cost Reagan a victory on this issue."

The switch may also have costs, however. After Zschau announced last month that he would vote by proxy for the administration aid package in the Foreign Affairs Committee, about 200 pickets came to his next major fund-raiser, in Santa Clara, to protest his turnabout. A Cranston aide said the senator, who has opposed such aid, "looks forward to debating the issue with any of the Republicans."