The Fair Oaks Republican is seeking the seat formerly held by Republican Sen. Dave Cox, who died in July at age 72.
Niello plans to make his announcement during a Republicans of River City meeting at 6:30 p.m. in the studios of Sacramento's KVIE Channel 6, according to an announcement from the club. Cox also announced his candidacy for the seat at a Republicans of River City meeting, according to the club.
Niello is finishing his third and final term representing the overlapping 5th Assembly District. Assemblyman Ted Gaines, R-Roseville, and Democrat Ken Cooley, mayor of Rancho Cordova, are also running in the Nov. 2 special primary.
If no candidate wins more than 50 percent of the primary vote, the top vote-getter from each party will advance to a Jan. 4 run-off. Republicans have a strong registration advantage in the district, which includes parts of Sacramento and Placer counties.
PHOTO CREDIT: Assembly Republican Roger Niello testifies as he presents the Republican budget plan to the Assembly budget committee, Tuesday Dec. 16, 2008. Brian Baer, Sacramento Bee.