Three-term Democratic Sen. Barbara Boxer is in her toughest re-election campaign ever, finding herself in a virtual tie with two of her possible Republican opponents while her disapproval rating is rising, according to a Field Poll released today.

Boxer trails former South Bay Rep. Tom Campbell 44 to 43 percent, and barely leads former Hewlett-Packard CEO Carly Fiorina, 45 to 44 percent, according to the survey of 748 likely voters this month. The poll's margin of error is plus or minus 3.7 percentage points.

Boxer leads Assemblyman Chuck DeVore, R-Irvine, 45 to 41 percent.

All three candidates have gained ground on Boxer since previous polls were reported.

Analysts said those comparisons largely reflect the national anti-incumbent feeling rather than support for the Republican candidates, who are not yet widely known. The poll found that nearly 60 percent of the respondents had "no opinion" about Campbell and Fiorina, and 78 percent felt the same about DeVore.

But voter sentiment about Boxer is turning negative. Her unfavorable rating jumped to 51 percent from 39 percent in January. And among the pivotal nonpartisan voters, her disapproval rating is 54 percent.

"The mood of the voters is really turning sour, and this is really a yes or no vote against Boxer," said Field Poll director Mark DiCamillo. "She has never been in this situation before, where she's standing in a strong headwind."

DiCamillo said he'd classify the race for Boxer's seat in the "toss-up" category.

Familiar story

Other recent polls tell a similar story. A new Wall Street Journal/NBC poll found that 4 in 5 respondents thought little of Congress, while voters were split on which party could better handle economic issues. And a Rasmussen Poll released Tuesday found Boxer leading each of her Republican opponents by single digits, with her lead over Campbell - her closest competitor - shrinking to two points.

Boxer's campaign team realizes the gravity of her situation. She has trailed in the polls during her re-election years before - she trailed Republican Matt Fong in the summer of 1998 - but this year she is also battling a national wave of voter frustration.

"Clearly, this is going to be the toughest campaign ever for Barbara Boxer," Boxer campaign manager Rose Kapolczynski said. "We need to reach out to those nonpartisan and undecided voters and tell Barbara's story."

Help from Obama

The campaign received some good news Wednesday: A White House official confirmed that President Obama would appear at a Los Angeles fundraiser for Boxer and the Democratic National Committee some time in April.

While the event is expected to net at least several hundred thousand dollars, Boxer campaign officials do not expect to start airing commercials until after the June 8 primary. Boxer is not expected to face any well-known Democrats in the primary.

"What I find significant is that Boxer is tied with three candidates who are markedly different from each other politically," said Henry Brady, a professor of political science at UC Berkeley and dean of the Goldman School of Public Policy there.

The pecking order in the GOP primary race didn't change much from Field's January temperature-taking. Campbell leads with 28 percent of the poll's support, followed by Fiorina at 22 percent and DeVore at 9 percent.

DeVore was the only candidate to have gained support, up from 6 percent in January.

Campbell delighted

A spokesman for Campbell, a pro-choice candidate who is the most moderate of the three, was delighted to have crossed the "psychological barrier" of finally leading Boxer in a poll. It will help fundraising for a candidate who only recently jumped to the Senate race from the race for governor.

"Barbara Boxer is not one of those Democrats who can separate herself from the national trend," said Campbell spokesman James Fisfis. "She can't say that she bucked her party. Where has she disagreed with her party before?"

"She's vulnerable, and she's vulnerable with independent voters," said Fiorina spokeswoman Julie Soderlund.

DeVore spokesman Joshua Trevino said the assemblyman's rating in the poll is "not where we want to be but we're moving in the right direction. It's OK. It's mid-March."