Gov. George Deukmejian called a special election for September 12 to fill the congressional seat vacated this month by Rep. Tony Coelho (D-Merced). If no candidate receives a majority, a runoff will be held between the top Republican and Democratic vote-getters on November 7, the same day as local elections. So far, Assemblyman Gary Condit (D-Ceres) and Modesto Mayor Carol Whiteside, a Republican, have announced their candidacies. By selecting Sept. 12, Deukmejian saved Whiteside from the embarrassment of having to pull down campaign signs she already has begun posting. Under state law, it is illegal to put up election signs more than 90 days before the vote. If the governor had picked a date any later in September, Whiteside's signs would have violated the law.