SAN FRANCISCO—California Democratic Sen. Barbara Boxer comfortably sailed to re-election in 2004, thanks to her state's reliable liberal voter bloc, but she faces headwinds this year as Republicans seek to tap the kind of voter sentiment in California that was behind the GOP's upset win in another left-leaning state.

The three major Republican candidates vying to unseat Ms. Boxer have been buoyed by Republican Scott Brown's surprise victory in Massachusetts last month, when he captured the Senate seat held for decades by Edward M. Kennedy. Mr. Brown defeated Democrat Martha Coakley, despite trailing her by a double-digit margin in the polls in the weeks before the Jan. 19 election in a state that, like California, usually leans Democratic.

"I think what Massachusetts demonstrates is it is possible for a Republican to win in a deep blue state," said Carly Fiorina, the former Hewlett-Packard Co. chief executive who is running in a June 8 GOP primary for a chance to face Ms. Boxer. The two other Republican challengers are former U.S. Rep. Tom Campbell and state Assemblyman Chuck DeVore.

Polls show Ms. Boxer is likely to have a fight on her hands to win a fourth term in November. Like other Democrats nationally, political observers said, the senator could face the wrath of voters concerned about the economy and government-spending initiatives that Ms. Boxer and others on Capitol Hill have championed. California also remains mired in an economic malaise, with an unemployment rate of 12.4%.

Ms. Boxer leads Mr. Campbell by four percentage points—45% to 41%—while holding an eight-point lead over both Ms. Fiorina and Mr. DeVore, according to a Jan. 27 poll by the nonpartisan Public Policy Institute of California. The poll, based on a telephone survey of 1,223 likely voters, has a margin of error of three percentage points. The survey also showed a rise in Ms. Boxer's disapproval rating to 44% among likely voters, from 39% in September.

"What we see this year in the California electorate is growing impatience with the California economy and unemployment," said Mark Baldassare, president of the San Francisco-based institute. "It has people in a very grouchy mood about incumbents."

The 69-year-old Ms. Boxer said she was taking nothing for granted. "Before Massachusetts, I was saying it would be a very tough election," she said, "and after Massachusetts, I am saying the same thing."

One fresh challenge for Ms. Boxer is from a new political force, the Tea Party movement. Comprising fierce fiscal conservatives who tend to side with GOP candidates, Tea Partiers generally detest the kind of government intervention that President Barack Obama and the Democrat-led Congress have embraced. They were instrumental in helping Mr. Brown get elected in Massachusetts, and leaders of the movement said another prime target was Ms. Boxer.

"I think the grass roots will determine what happens in this election," said Mark Meckler, cofounder of the National Tea Party Patriots and a resident of Grass Valley, Calif. "And Boxer does not share my values, which is fiscal responsibility."

Ms. Boxer said she was taking the Tea Partiers seriously, but disputed the assertion that she was a reckless spender. She added that many Tea Partiers embraced conservative ideals, including opposition to abortion, that she and most Californians reject. "Those people have been trying to get me out of office forever," she said.

Of the Republican candidates, Mr. DeVore, a staunch conservative from Orange County, is a favorite among the Tea Partiers, and said he can ride their support to victory—even though he polls a distant third in the primary. "This is one of those moments in American politics that is a populist moment," the 47-year-old Mr. DeVore said.

His rivals from Silicon Valley—Mr. Campbell, 57, and Ms. Fiorina, 55—said they expect to garner the support of the Tea Partiers, too. "I share values which are important to primary voters, such as the fact I am not a professional politician," Ms. Fiorina said. Mr. Campbell, who holds a doctorate in economics, said, "What really benefits me is the focus on economics. It's the economy, stupid."

Republican strategists said toppling Ms. Boxer won't be easy. In 2004, she received about seven million votes—a record for a statewide contested office in the U.S. She is also a prodigious fund-raiser, already having amassed about $7 million in a campaign war chest her advisers say they expect will grow to $20 million. And as chairman of the Environment and Public Works Committee and close ally of Mr. Obama, she can count on top Democrats from around the country to stump for her.

"It's doable, but she is a very determined and committed candidate who wants to protect the power she has accumulated," said Ken Khachigian, an attorney and Republican strategist in San Clemente, Calif.
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