An "ominous" new poll shows Sen. Barbara BoxerBarbara BoxerTime is now to address infrastructure needs Tom Steyer testing waters for Calif. gubernatorial bid Another day, another dollar for retirement advice rip-offs MORE’s (D-Calif.) approval rating tanking as Vice President Joe BidenJoe BidenBiden tells LGBT community to hold Trump accountable Budowsky: Dems madder than hell The Hill's 12:30 Report MORE visits the state to help her raise money.

Biden will be in Silicon Valley and Los Angeles on Thursday to help Boxer gather contributions for her reelection run against Republican Carly Fiorina.

A new Field Poll gives Boxer some of the lowest approval ratings of her three terms in office. The poll also found  the race between Boxer and Fiorina is a statistical toss-up.

"She is vulnerable," Field Poll director Mark DiCamillo told the San Francisco Chronicle. "This is very ominous for her."

In the June 22-July 5 survey, Boxer led Fiorina among registered voters by the slim margin of 47 to 44 percent. Boxer was viewed unfavorably by 52 percent of respondents, while 43 percent disapproved of her performance and 42 percent approved.

Among the "likely" registered voters surveyed, 48 percent disapproved of Boxer's performance and 42 percent approved.

Fiorina's campaign trumpeted the results.

"Today's Field Poll results prove that California voters are ready to fire Barbara Boxer," Fiorina spokeswoman Julie Soderlund said in a statement.

The Field Poll of 1,390 people had a margin of error of 3.2 points.