The survey, conducted by theJames Irvine Foundation and the Public Policy Institute of California, found Harris leading the two-way, all-Democratic contest by 22 points, 42 percent to 20 percent, among likely voters. Twenty percent were undecided and 18 percent said they would not be voting. Harris led Sanchez by 7 points in the September edition of the poll.
Harris and Sanchez were the top two vote-getters in California's open primary in June. Under the state's election rules, the top two finishers advance to the general election, regardless of party.
The poll also found Democratic presidential nominee Hillary Clinton leading her Republican rival Donald Trump by 26 points, 54 percent to 28 percent, in the Golden State. Libertarian Gary Johnson and the Green Party's Jill Stein trailed with 5 percent each.
Harris has largely received the backing of the national and state Democratic establishment. She won the endorsement of President Barack Obama, Vice President Joseph R. Biden Jr., California Gov. Jerry Brown and both of California's senators, Dianne Feinstein and Barbara Boxer, whose retirement after this term set up the open Senate seat.
Harris leads among white and African-American voters, while Sanchez holds the edge among Latino voters.
The poll also showed a majority of Democrats favor Harris but 45 percent of Trump supporters and 36 percent of California Republican voters said they would not vote in the Senate race.
The poll surveyed 1,024 likely voters by landline telephone and cellphone from Oct. 14-23. The margin of error was 4.3 percentage points.