Boxer and Fiorina are virtually tied in recent polls and national political handicappers now believe it's a tossup whose outcome could determine whether Republicans take control of the Senate this year.
In years past, national political organizations have shied away from committing large sums to California's Republican Senate hopefuls due to the state's high campaign costs but that may change if Boxer looks vulnerable and if the California Senate race could have national impact.
"We're endorsing Carly because she understands the challenges businesses are facing and will stand up for the people of California during these tough economic times," said Bill Miller, the U.S. Chamber's senior vice president and national political director.
Miller is joining Fiorina on a two-way swing through the state today to highlight economic issues. The chamber has pleged to spend $75 million this year to support congressional and senatorial candidates.