Obama has recorded a 60-second radio spot praising the three-term senator's work to create jobs and improve the economy.
"I've stood shoulder to shoulder with Barbara Boxer through the tough fights. I believe California needs her and I know I need her," he says in the ad, which you can listen to here.
This week's visit marks the third time Obama has traveled to the Golden State to aid Boxer, who is locked in a tough race with Republican businesswoman Carly Fiorina.
Fiorina's campaign countered by pointing to California's high unemployment rate as evidence that Boxer is not a job creator.
"Barbara Boxer should be ashamed of herself for putting the President of the United States up to participating in this latest installment of her desperate and deceptive advertising campaign to save her political career," Spokeswoman Andrea Saul wrote in an e-mail.
This post was updated at 6 p.m. with a statement from the Fiorina campaign.
PHOTO CREDIT: President Barack Obama hugs Sen. Barbara Boxer, D-Calif., at a fundraiser for the Democratic National Committee and Sen. Barbara Boxer, D-Calif., at the California Science Center in Los Angeles, Monday, April 19, 2010.(AP Photo/ Alex Brandon).