BURBANK, Calif. » If Republicans are to have a serious chance of capturing control of the U.S. Senate in November, they must win in traditionally Democratic states like California, where Sen. Barbara Boxer, a three-term Democrat, is showing signs of vulnerability.

But before Republicans get a clear shot at Boxer, they will have to overcome deep divisions within their own party—divides that reflect both the grass-roots energy surging through the conservative movement and the tensions between the party's moderate and conservative wings.

There are certainly more vulnerable Democratic Senate seats in the country, but early polls in California suggest that Boxer is facing what could be the toughest election of her career. Her difficulties in a state that has for 20 years proved reliably Democratic in national elections suggests how the pendulum has swung against Democrats in just a year. Her potential problems are a function more of this political climate than of any position or vote she has taken.

Still, for Republicans, this could end up being a repeat of a play they have seen before: a promising opportunity escaping them in a state where Democrats have the edge of 1.5 million more voters registered.

Three Republican candidates are in a lacerating battle for their party's nomination: Carly Fiorina, the wealthy former chief executive of Hewlett-Packard; Tom Campbell, a former member of Congress who fits the moderate profile for the kind of Republican who has won statewide contests in the past; and Charles S. DeVore, a state assemblyman who is presenting himself as the Tea Party candidate.

It is hard to see how Republicans could win control of the Senate without toppling Boxer. Democrats control the chamber 59-41; Republicans need 10 seats to take control, since in an evenly divided Senate, Vice President Joe Biden would cast the tie-breaking vote in favor of the Democrats. Eight Democratic seats are considered vulnerable, but Republicans must hold on to at least four of their own seats that appear vulnerable. Even if Republicans won all those highly contested races, they would still need two more victories, and at the moment California might be their best chance to expand the battlefield.

John J. Pitney Jr., a political science professor at Claremont McKenna College, said Boxer's previous easy victories in Senate races could be misleading, since they took place in what were more favorable political environments. “;She may be more vulnerable than it seems at first,”; Pitney said.

Democrats fear that a Supreme Court decision allowing corporations to spend freely on political campaigns could encourage a flood of advertising against Boxer.

She is clearly aware of the threat to her re-election and has moved forcefully to deal with it. She has already raised nearly $11 million.

“;If you're asking me if Republicans have a chance to beat me, I'm going to answer this way: I never take any election for granted,”; Boxer said in an interview. “;It is a tough year. I am going to focus; I am going to work my heart out.”;

Campbell is a self-described fiscal conservative who supports abortion rights and same-sex marriage. “;He would have more appeal to moderate voters than any other Republican nominee—other than Arnold or Pete Wilson—has had here for the past 25 or 30 years,”; said Bill Carrick, a longtime Democratic consultant in the state, referring to Gov. Arnold Schwarzenegger and Wilson, the former California governor.

But the positions that might help Campbell in a general election are a burden in the Republican primary. And the hurdle is not only on social issues. When Campbell was finance director for Schwarzenegger, he supported temporary tax increases to deal with the state's worsening fiscal crisis, and Fiorina and DeVore have criticized him for that.

“;Tom Campbell has a distinctly different point of view than I do,”; Fiorina said. “;He believes the way to close budget deficits is to raise taxes. I think that's the wrong approach.”;

Campbell has also come under attack from his rivals for his ties to a professor who aided Palestinian militants and for being, by their account, insufficiently supportive of Israel. Campbell has said that he was unaware of the professor's activities and that he supports Israel.

Campbell suggested that Fiorina and DeVore may split the conservative vote in the June 8 primary, allowing him to slip through. He has also said some conservatives might overlook their differences with him in their hunger to oust Boxer.

“;She is particularly vulnerable to me,”; Campbell said in an interview. “;I'm pro-choice. It's going to be the first time she's ever had to run against a candidate with my profile. It's a huge advantage for me.”;

Fiorina was at one point viewed by Republicans as a formidable candidate because of her wealth (she has put $2.5 million into her campaign) and a compelling personal story: she rose from receptionist to chief executive and is also a breast cancer survivor.

But she has not had an easy transition from business executive to candidate. Ms. Fiorina suggested the other day that California file for bankruptcy; that is legally not possible. Her campaign produced a widely ridiculed Internet attack video portraying the mild-mannered Campbell as a wolf, with glowing red eyes, in sheep's clothing. (The point was that he was not really a fiscal conservative.) DeVore has been unsparing in his attacks on her.

“;She has some personal wealth, but she was fired from Hewett-Packard,”; he said. “;She only voted six times in her life. She has a condo in Georgetown at the Ritz-Carlton that is 13,000 square feet bigger than my house—and she confessed she got it with the golden parachute money she got when she was fired. These sort of flaws will be played up significantly by Boxer.”;

DeVore may be the wild care in the race. He has won the backing of an influential conservative political action committee created by Sen. Jim DeMint, R-S.C.. And his candidacy will offer a test of the strength of the Tea Party movement in this state. A poll taken in California in January found that 70 percent of Republicans had heard of the Tea Party movement, with 52 percent of them identifying with its ideas somewhat or “;a lot.”;

But DeVore, who is known as Chuck, is struggling to raise money, and remains barely known by most Californians. He said he hoped that as his profile rose, he would draw a wave of Tea Party contributions from across the country.

“;This is eminently winnable for all three Republicans,”; DeVore said of the Boxer race, “;but I would argue, counterintuitively, that I am the most dangerous threat. Because I am the guy who is the toast of the Tea Partiers.”;