Voters in 12 Northern California counties are choosing a successor to a Republican state senator who died in office last year.

Republican Assemblyman Ted Gaines of Granite Bay is competing in Tuesday's special election with Democrat Ken Cooley, the mayor of Rancho Cordova. They emerged as the top vote-getters in their parties after the special primary Nov. 2.

Gaines is favored because Republicans hold an 11-point advantage in voter registration. He already represents nearly a third of the 1st Senate District, which had been represented by the late Sen. Dave Cox, a Republican from Fair Oaks.

The district stretches from the Oregon border south to Mono Lake and includes all or parts of Alpine, Amador, Calaveras, El Dorado, Lassen, Modoc, Mono, Nevada, Placer, Plumas, Sacramento and Sierra counties.