But if the opportunity appears ripe for a Republican takeover of a seat Boxer has held for almost two decades, less clear is which GOP candidate has the best chance of pulling it off. Is it former congressman Tom Campbell, whose stance favoring abortion rights would deprive Boxer of an issue she's used to batter past GOP opponents? Or would Carly Fiorina — the first female after a succession of male opponents that Boxer has dismantled — give the senator the biggest fits? And then there's conservative state Assemblyman Chuck DeVore, a favorite of party activists, who may be best able to harness "tea party" anger at Washington.

Polls show Campbell with a lead over Fiorina ranging from low single digits to double digits heading into the campaign's final month. DeVore has consistently lagged in third place, although one recent poll showed him within striking distance of second. Still, with the bulk of TV and mail advertising yet to come, and a plurality of voters still up for grabs, the race remains highly fluid.

With the June 8 primary just a few weeks away, here's a look at the strengths and vulnerabilities each of the GOP contenders would bring to a general election matchup with Boxer, informed by interviews with campaign aides and outside political observers.

Tom Campbell

Best case: Campbell's biggest weakness in the primary campaign — his liberal positions on social issues such as abortion and gay marriage, and a pragmatic streak when it comes to taxes — could be among his greatest strengths in a general election. Simply put, he is less ideological than Boxer's past opponents, and would likely attract more support from independent voters who tend to sway general elections.

Take abortion rights, which Campbell favors. Boxer has used the abortion issue in past races to rally women and paint her GOP opponents as extremists. That wouldn't be possible with Campbell.

As a Ph.D. in economics who was once ranked among the most fiscally conservative members of Congress, Campbell would also have a compelling case to make that he'd be a strong steward of the nation's finances — a potentially potent advantage against the liberal Boxer. His willingness to endorse tax increases (as he did last year as a candidate for governor, to help stave off devastating cuts to public schools), which has created a huge target for his primary opponents, would likely fade in the general election. And he's the only GOP contender with elected experience in Washington.

Cause for concern: Campbell has run twice for Senate and lost both times — in a 1992 GOP primary (Boxer won the general election), and in 2000, when he was crushed by Democratic Sen. Dianne Feinstein. Those losses, combined with his decision to abandon the governor's race early this year, raise doubts about his viability as a statewide candidate.

There are also questions about whether Campbell — a mild-mannered law professor who's about as nice a politician as you're likely to find — has the toughness to go toe-to-toe with Boxer, a notoriously scrappy, no-nonsense campaigner.

Money is another potential weakness. Though he's raised a decent amount of money in the primary, it pales in comparison to Boxer's multimillion-dollar haul. Finally, Campbell has come under attack for voting to cut economic aid to Israel and for defending a Florida professor aligned with the Palestinian Islamic Jihad. Those problems could continue to haunt him in the general election.

Upshot: His political profile — mostly conservative on fiscal issues, liberal on social issues — would give him a leg up with key independent voters. But his professorial demeanor and fundraising prowess would be major question marks in a matchup against the scrappy and well-funded Boxer.

Carly Fiorina

Best case: It would be unfair to Fiorina to suggest that her gender would be one of her main assets in the general election, but there is no question that it would present a unique challenge for Boxer — who stormed into the Senate in 1992 with a full-throated appeal to women and has followed that pattern against male challengers since. If Fiorina can manage to peel off even a modest amount of Boxer's female support, it would make the senator's path to victory that much narrower.

Gender aside, Fiorina has a number of attributes that make her a potentially tough opponent for Boxer. She is an engaging public speaker with a commanding stage presence. She does not shy away from a fight. And she has a compelling career arc, having risen from law school dropout to secretary to first female CEO of a Fortune 20 company.

But perhaps most threatening to Boxer is Fiorina's personal checkbook. Fiorina has the wherewithal to pump millions of her own dollars into the campaign, at least partially offsetting Boxer's natural fundraising advantage. As a former surrogate for John McCain's presidential campaign in 2008, Fiorina is also plugged into the fundraising network of the national Republican Party establishment.

Cause for concern: Fiorina's record at HP makes her a sitting target for political opponents. At a time of record unemployment in California, she will have to explain why she laid off almost 33,000 employees, sent jobs overseas and walked away with a $21 million severance package after she was fired. Fiorina's ability to counter that narrative with a more positive story — of, by her account, a visionary who revitalized a struggling company — will take sustained advertising, which is why money is so important.

Other potential weaknesses that Boxer might exploit include Fiorina's opposition to abortion rights and her failure to vote in most elections, which fuels suspicions that her foray into politics is more about ego gratification than some larger cause.

Upshot: Personal wealth, charisma, and gender make her a potentially formidable opponent, but her background at HP will be a difficult sell.

Chuck DeVore

Best case: As the GOP candidate with unquestioned conservative credentials, DeVore is perhaps best positioned to tap into Republican voter anger at Washington. If he can somehow manage to squeak by Campbell and Fiorina in the primary, his best-case scenario would be something along the lines of what has unfolded in Florida this spring, where conservative Marco Rubio has stormed into serious contention for a Senate seat on the back of the tea party movement. If DeVore could show he had a legitimate shot at beating Boxer, that could boost his national fundraising efforts, which have yet to take off. DeVore also has a military background and worked in the Defense Department under President Reagan.

Cause for concern: DeVore is probably the candidate Boxer would most welcome as her opponent. His staunch conservative profile on issues ranging from abortion to immigration would allow her to argue that he is outside the political mainstream in California, a case she has made successfully against past opponents.

Upshot: Ideologically, he's the purest of the GOP bunch, but that could spell trouble in a general election.

Of course, Boxer would bring her own set of strengths and vulnerabilities to the race. Her liberal voting record, particularly at a time of growing concern over government spending, will hand plenty of ammunition to her Republican opponent. On the other hand, California remains a solidly Democratic state.

"There are a lot of Republicans who are probably licking their chops," said Darry Sragow, a longtime Democratic strategist who is not involved in the race. "But Barbara Boxer is a tough and fearless campaigner, and she's never taken re-election for granted."

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