He urged supporters to directly donate to the DeVore campaign.
"California is ready for a turn back to freedom," DeMint said. He said DeVore has a "record straight up the way mine is" and applauded the candidate for flying to Washington to meet him.
The news comes as Fiorina officially entered the race via an editorial in the Orange County Register where she asked "those who expect more from our government to join me in making a change for the better."
Erick Erickson, editor of RedState.com, also appeared on the call and noted that DeVore served in the Reagan White House. He criticized Fiorina for backing the stimulus and noted recent polling showing both of the candidates are within 10 points of Boxer.
DeMint targeted Boxer in her role as chairman of the Environment and Public Works Committee and said she is "surprisingly vulnerable for a Democrat running in such a blue state."
He said Boxer would "add to the cost of living" with her climate bill and would "cost Americans a lot of jobs."
The Republican primary is June 8, 2010.
The group took a snap poll via telephone of the callers on the line and 78 percent said they would back DeVore while just 5 percent said they supported Fiorina. The rest were undecided.