Robert W. Naylor, chairman of the California Republican Party, announced that President Reagan will send a letter to voters endorsing Assemblyman Wayne Grisham (R-Norwalk) for the state Senate in a special May 12 runoff election against Democrat Cecil N. Green, a Norwalk city councilman. At a Sacramento press conference, Naylor said the Reagan endorsement--unusual for a state legislative contest--demonstrates that Republicans regard the election as a critical first step toward taking control of the state Senate in advance of legislative reapportionment which will take place after the 1990 election. Frank Lavin, director of the White House Office of Political Affairs, confirmed the endorsement, saying "the President is indeed for Wayne Grisham." In addition, Naylor said that Gov. George Deukmejian, who stumped for Grisham before the March 17 primary election, plans to appear at two fund-raisers on Grisham's behalf.