Sen. Dianne Feinstein (D-Calif.) might be vulnerable to a Republican challenger.

According to a new Field poll, just 41 percent of California voters support reelecting the longtime centrist incumbent, who won her last two reelections with ease, while 44 percent oppose reelecting her. Feinstein's job approval rating remains positive, but at a dangerously low 41 percent approving, with 39 percent disapproving.

The poll is the latest worrisome sign for Senate Democrats. Still, it it is unclear whether California Republicans could field a serious challenger to Feinstein who could raise the multiple millions of dollars needed for a campaign in the highly expensive state, and having GOP candidates spend multiple millions of dollars in 2010, a great Republican year, Democrats held on to both a gubernatorial and Senate seat by double-digit margins.

Feinstein's numbers are likely being dragged down more by a general disapproval of Congress than any failing of her own: Just 9 percent of California voters approved of Congress in the poll.

Her numbers are much worse now than they were at the same point in her last serious reelection. In August 1993, 53 percent of voters backed Feinstein's reelection in a Field poll. She went on to squeak out a win with 47 percent of the vote against billionaire Michael Huffington the following year.

Feinstein also could be in trouble monetarily: Her campaign treasurer was just arrested earlier this month for embezzlement, and she told supporters that her campaign fund may have been "wiped out," according to the Los Angeles Times.

Republicans Carly Fiorina and Meg Whitman, the two Republican standard-bearers from 2010, have the personal resources to match Feinstein, but it is unclear whether either will want to run again after their big-margin losses last election. Some California House members whose districts were made much more difficult to hold could also decide to challenge her: Rep. David Dreier (R-Calif.), who no longer has a Republican-leaning district to run in, could prove a tough candidate.

The poll of 1,001 registered was conducted from Sept. 1-12, with a margin of error of plus or minus 3.2 percent.