By Sharon Bernstein

SACRAMENTO, Calif. (Reuters) - California voters on Tuesday are expected to choose two Democrats to face off against each other in the race to succeed Barbara Boxer in the U.S. Senate, shutting out Republicans in a sign of that party's diminished support in America's most populous state.

State Attorney General Kamala Harris and U.S. Representative Loretta Sanchez will likely meet in what would be the state's first-ever single-party Senate election under a 2010 California law advancing the top two primary vote-getters to the general election, regardless of party affiliation.

In recent opinion polls ahead of the state's primary on Tuesday, Harris, 51, has led the crowded field of 34 candidates. None of the Republican contenders comes close to Sanchez, 56, who is firmly in second place. Boxer, 75, a Democrat, is retiring after 24 years in the Senate.

Republicans hold no statewide offices in California and represent about 27 percent of registered voters. The state, home to former Republican Presidents Richard Nixon and Ronald Reagan, last elected a Republican to the U.S. Senate in 1988.

Harris is the daughter of two college professors. Of African-American and Indian descent, she was raised amid civil rights activism in Berkeley and Oakland, eventually becoming a prosecutor in San Francisco. She was elected attorney general in 2010 and 2014.

Harris has run a tightly controlled campaign, touting her role negotiating a settlement with big banks over the mortgage meltdown and using her position as the state's top prosecutor to fight human trafficking, cyber crime and other issues.

Sanchez, who represents Orange County near Los Angeles, has served in Congress since 1997. Her win in the once staunchly Republican county was an early sign of California's demographic shift, as the state became more ethnically diverse and far more Democratic.

COULD GO NEGATIVE

Such a matchup could easily go negative, experts said, straining relationships within a party that are already bruised by the tense competition between former Secretary of State Hillary Clinton and U.S. Senator Bernie Sanders for the Democratic presidential nomination for the Nov. 8 election. Clinton and Sanders also face off in California on Tuesday.

"The fact that they're both in the same party doesn't mean they're going to treat each other with kid gloves," political analyst Sherry Bebitch Jeffe said of Harris and Sanchez.

"When ideological and policy differences aren't as great, it's only natural for the candidates to focus on personal issues," said Dan Schnur, who heads the Unruh Institute of Politics at the University of Southern California.

Sanchez, seeking to become the state's first Latina U.S. senator, has raised less money than Harris and trails her by about 8 percentage points in polls. But factors including a recent surge in voter registration among Latinos will make her a strong competitor in the general election, analysts said.

She has positioned herself more to the political center than her opponent.

With support from Latinos, independents and Republicans, Sanchez could win in November despite Harris' advantages, Schnur said.

"We're prepared for any turn in the race," said Harris spokesman Nathan Click. A Sanchez campaign spokesman did not respond to a request for comment.

(Reporting by Sharon Bernstein; Editing by Peter Cooney)