Formal candidacy papers cannot be filed until Aug. 31, but Cooley said he has decided to run in the 1st Senate District, which stretches from Sacramento and Calaveras counties to the Oregon border.
Incumbent Republican Assemblymen Ted Gaines of Roseville and Roger Niello of Fair Oaks also are expected to run in the Nov. 2 primary. Gaines formally has announced his candidacy.
District voters can vote for candidates of any party in the primary, but if no candidate receives more than 50 percent of the vote, the top finisher of each party will square off Jan. 4.
Cooley currently works as principal consultant for the state Senate Committee on Banking, Finance and Insurance.
The 57-year-old Rancho Cordova resident has been a councilman since the city's incorporation in 2002. He served as mayor in 2005, mayor pro tempore in 2004, and vice mayor in 2009.
Cooley is a former vice president of the League of California Cities and former vice chairman of the state's seismic safety commission. He ran unsuccessfully in the 2002 Democratic primary for the 10th District Assembly seat, which ultimately was won by a Republican, Alan Nakanishi.
Republicans outnumber Democrats by a wide margin in the 1st Senate District, 43 percent to 34 percent, but Cooley said he considers himself a moderate Democrat whose strong family values as a churchgoer and former scoutmaster could appeal to voters of all political stripes.
A Democrat could win the Senate seat, perhaps even in the Nov. 2 primary, if it's "a strong candidate who connects with people and has a good personal reputation - particularly if there's an ugly Republican primary that could turn people off, which is always possible," Cooley said.
Cooley and his wife, Sydney, have two adult sons.
PHOTO CREDIT: Ken Cooley in a 2002 file photo. Scott Lorenzo, Sacramento Bee.