Is Barbara Boxer in serious trouble in California?

California Sen. Barbara Boxer is quickly becoming one of the Democrats’ most vulnerable incumbents ahead of the November midterms. A new Field Poll finds Boxer narrowly leading former Hewlett-Packard chief Carly Fiorina in her bid for re-election, 47 percent to 44 percent. That’s slightly up from the last Field survey in March, when the Boxer led by just a point, but it’s a huge downward lurch from the 15-point lead Boxer held in the race in January.

A year ago, Boxer had been considered a safe bet for re-election. But she’s now facing many of the same difficulties that are dogging her colleague Harry Reid in his re-election campaign in Nevada: Voters are increasingly unhappy with her job performance, and personally, they don’t like her.

Boxer’s likability ratings have dropped 9 points since January. Just 41 percent of Golden State voters say they have a favorable opinion of her, compared to 52 percent in the unfavorable camp. Perhaps most unnerving for Boxer, however is her job approval numbers. Only 42 percent of likely voters approve of the job she’s doing — the lowest rating Boxer has received since she was first elected to the Senate in 1992. By comparison, her approval was 48 percent in January. According to the poll, 48 percent now disapprove of her job performance, while 10 percent are undecided.

One plus for Boxer is that she leads her GOP challenger among self-described moderates, 53 percent to 34 percent — a swing vote group that Fiorina desperately needs to win to overcome the huge advantage in voter registration that Democrats hold in the state. Boxer also holds double-digit leads among three major ethnic groups, Latinos, Asian Americans and African-Americans — collectively a third of the state’s voting population. She also leads among women.

Yet Boxer faces at least one major disadvantage compared with her previous elections: There’s no race at the top of the ticket driving turnout. In 1992 and 2004, she ran during presidential election years, and in 1998, her campaign got traction from the huge turnout in the state’s gubernatorial race. Even with an open gubernatorial race in California this year, there's been no discernible spike in voter enthusiasm for the 2010 cycle. Polls in California reflect the overall trend that voter surveys show nationally: Republicans are much more excited about the November midterms than Democrats are.

Boxer is hoping to turn that momentum around. She’s campaigning with Vice President Joe Biden in the state Thursday; and President Obama, who has appeared on her behalf three times so far this cycle, will probably visit again before November.