California Sen. Barbara Boxer (D) has increased her lead over Republican Carly Fiorina in the latest Field Poll released Friday.

Boxer has a six-point lead over her challenger, 47 percent to 41, with 12 percent still undecided, in the new survey. The Democrat led by only three points in the July poll.

This race is shaping up as a referendum on Boxer's tenure. Two-thirds of Fiorina supporters said that their preference is more a vote against Boxer — a protest vote — with only 31 percent saying it was a vote for Fiorina. Moreover, 28 percent of respondents still have no opinion of Fiorina.

And of those who do, Fiorina generates a mixed response, with 34 percent of voters holding a positive impression and 38 percent a negative assessment.

Fiorina’s first TV ad of the general election, which was released this week, did little to expand voters' impression of her. The spot focused exclusively on Boxer and her "arrogance."

The Field Poll was conducted Sept. 14-21 and surveyed a random sample of 857 registered voters, of whom 599 were considered likely to vote in the midterm election. 

Fiorina's camp noted that the poll was being done as Boxer was up statewide with an ad that hits the Republican for her tenure at the head of Hewlett-Packard. In the close of the 30-second spot, the announcer says of Fiorina, "outsourcing jobs, out for herself."

Moreover, said Fiorina spokeswoman Andrea Saul, "after spending millions of dollars on baseless, deceptive and unanswered attack ads against Carly, she failed to gain any support and only temporarily moved 3 percent of the vote to undecided."

This is the TV ad that Boxer released earlier this month.

--Updated at 1:01 p.m.