California voters are giving U.S. Sen. Barbara Boxer some of the lowest approval ratings of her career, as the three-term Democrat is in a statistical dead heat against first-time GOP office-seeker Carly Fiorina, according to a new Field Poll released today.

Boxer leads Fiorina, the former Hewlett-Packard CEO, 47 to 44 percent, according to Field's survey of 1,390 registered voters, 1,005 of whom were considered likely to vote in November. The poll, conducted June 22-July 5, has a margin of error of 3.2 percentage points among likely voters. For Boxer's job performance ratings among registered voters, the margin of error is 2.8 percentage points.

Boxer's slight numerical lead masks potentially serious problems for the senator, starting with how 52 percent of the respondents hold an unfavorable view of her.

At the same time, her job approval rating is among the lowest that Field has measured for her since she was first elected to the Senate in 1992: 43 percent of registered voters disapprove of her performance while 42 percent approve. Among likely voters, 48 percent disapprove and 42 percent approve.

While voters' responses to a pollster in July may change by the time they vote in November, the new poll shows that whether voters love or hate her, 93 percent hold an opinion about Boxer.

"She is vulnerable," Field Poll director Mark DiCamillo said. "This is very ominous for her."

One of Boxer's more vexing problems, analysts say, is that opposition to her is not just about her. She has become an avatar for broader voter frustrations about the struggling economy, President Obama and the growth of the federal government.

"It's a reflection of the effectiveness of a Republican strategy to characterize Sen. Boxer as everything that's wrong with the government," said Larry Berman, a professor of political science at UC Davis. Sen. Harry Reid, D-Nev., another longtime Democrat facing a tough re-election challenge, faces a similar predicament, Berman said.

When the economy is struggling, DiCamillo said, "the voters tend to take it out on the incumbents."

Boxer hasn't run from her close relationship with the White House.

She barnstormed California Tuesday, touting her support of the Obama administration's $862 billion stimulus package that the Senate narrowly approved in early 2009. Vice President Joe Biden will headline fundraisers for Boxer in Silicon Valley today and in Los Angeles on Friday. Obama has visited the state twice this year for Boxer fundraisers.

Fiorina, meanwhile, remains unknown to 37 percent of voters, the poll found. She emerged from her three-way GOP Senate primary with "a bit of a halo effect," DiCamillo said, as 34 percent of the respondents hold a favorable image of her - up from 20 percent in March when she was battling former South Bay Rep. Tom Campbell and Assemblyman Chuck DeVore of Irvine for the GOP nomination.

The poll found that 29 percent of the respondents had a negative image of Fiorina, up from 22 percent in March.

What has helped Fiorina, analysts said, is that her primary opponents didn't have much money to spend on ads that would have driven up her negative ratings.

"All three of the Republican (Senate candidates) were running against Boxer, by-and-large," DiCamillo said.

One bit of good news for Boxer is that she continued to lead among independent voters, 47 to 39 percent, with 14 percent undecided. And she is helped by the fact that 45 percent of California voters are Democrats, 31 percent are Republicans and 20 percent are not registered to a major or minor party.

But Boxer's success with independents may also be because Fiorina ran as a conservative candidate to win her party's nomination.

The challenge for Fiorina is to capture the independent vote without moving too far to the center and alienating her Republican base, said Jason McDaniel, an assistant professor of political science at San Francisco State University.

While the general perception is that an incumbent polling less than 50 percent is at risk, McDaniel said Boxer tends to do better in polls as the race goes on and then outpaces polls in elections. That happened in her 1998 and 2004 re-election campaigns, he said.

Without a well-funded Democratic primary opponent, Boxer has only recently begun to campaign.

"We haven't seen the big money start rolling into this campaign yet," Berman said. "And it will."