His plan is to "shake down the liberal suits," as SF Gate's Joe Garofoli put it, and build up Boxer's campaign war chest.
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"You don't have a chance if you don't have money, but once you reach a certain threshold, you have to get the message out effectively," says Bryan Adams, political science professor at San Diego State University.
He doesn't think Biden will make anyone vote for Boxer -- but he will definitely be an effective fundraiser.
"Biden's viewed as a senior statesman, he has personality, and the race is much closer than Boxer would like to think, " says SDSU professor Ronnee Schreiber, who's written a book about conservative women in politics. "She's nervous."
Even in this atmosphere of anti-incumbency, "it's Boxer's race to lose, unless she makes a major misstep," Adams said. "She has name recognition, she's experienced, all of the skeletons are out."
Fiorina treated Biden's visit like an act to help a desperate candidate, calling it the "fourth rescue mission to California for Barbara Boxer," referring to President Obama's two visits in April and May.
Fiorina does have a big-name backer, too. Sarah Palin whipped up support for Fiorina during the primary race with an endorsement on Facebook, and Fiorina is sure to lean on the former VP candidate again.
Biden and Palin, together again?