Sen. Jenny Oropeza was leading Republican challenger John Stammreich 58.4 percent to 35.7 percent, according to the latest vote count, The Sacramento Bee reported.
If the results hold up, Gov. Arnold Schwarzenegger will call a special election.
Oropeza, who represented a district in Southern California's Long Beach area, died Oct. 20 at age 53 of complications from a blot clot in her abdomen. Because she died so close to the election, her name stayed on the ballot.
The campaign became heated on its final day. Stammreich and the state GOP filed a complaint accusing the Democratic Party and secretary of state of using a final-hour mailer to "illegally influence" the voting public, the Daily Breeze reported.
The lawyer for the state Republican Party, Chuck Bell, did not rule out a challenge to the election results based on possible ""illegal conduct affecting the vote," the newspaper said.