US Republican presidential candidate Donald Trump says he will not debate Democrat Bernie Sanders ahead of California's June 7 primary.
"Based on the fact that the Democratic nominating process is totally rigged ... it seems inappropriate that I would debate the second place finisher," Trump said on Friday in a statement.
Sanders expressed disappointment at the development on Friday.
"I heard that he was going to debate me, then I heard that he was not going to debate me, then I heard that he was going to debate me. Now you're telling me that he is not going to debate me. Well, you know, I hope that he changes his mind again," Sanders said in a video clip posted on an ABC News Twitter account.
Trump suggested broadcast networks were unwilling to go along with his demand that at least $US10 million ($A13.86 million) raised from the encounter be donated to charity.
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"I'd love to debate Bernie," he told a rally in Fresno, California.
"But the networks want to keep the money for themselves."
