* Fiorina leads Boxer, 47 percent to 42 percent
* Results show Boxer losing support among independents
By Dan Whitcomb
LOS ANGELES, Aug 12 (Reuters) - Republican Carly Fiorina has built a five-point lead over incumbent Democrat Barbara Boxer in their race for the U.S. Senate in November, a poll released on Thursday shows.
Fiorina, a Republican former Hewlett-Packard CEO making her first run for political office in a state considered reliably Democratic, is beating Boxer by a margin of 47 percent to 42 percent, according to the SurveyUSA poll.
Republicans would love to knock off Boxer, a powerful liberal voice in the Senate, in a year when voter discontent over joblessness and the economy has left Democrats and incumbents vulnerable, and the campaign in California is being closely watched by both parties.
Political analysts believe Fiorina, 55, may pose the toughest challenge yet for Boxer, 69, who was first elected to the Senate in 1992 after five terms in the House of Representatives and has since been unbeatable.
SurveyUSA, which sampled 602 likely California voters between Aug. 9 and Aug. 11, found Fiorina had increased her lead by several percentage points in the last month.
The SurveyUSA poll conducted in July showed Fiorina edging Boxer by a slim margin of 47 percent to 45 percent.
Poll directors said their survey indicated Fiorina's support was largely unchanged among most voting segments, while Boxer had lost support among men, younger voters and critical independents.
Fiorina had also gained significant ground among voters in the Central Valley agricultural hub, a conservative stronghold in an otherwise blue state.
The survey, which has a margin of error of plus or minus 4.1 percent, found Democrat Jerry Brown and Republican Meg Whitman essentially in a dead heat in their race for California governor.
Proposition 19, which would legalize possession of small amounts of marijuana in California, was passing 50 percent to 40 percent -- unchanged since the July survey. (Editing by Jerry Norton)