Bernie Sanders had a ready response to Donald Trump's change of heart over a proposed debate.

"Mr. Trump what are you afraid of? Why do you not want to see a debate here in California -- and obviously all across this country -- on why you think it's a good idea to be trying to divide up our people, to be scapegoating Mexicans and Latinos and Muslims and women and veterans and African-Americans," Sanders said in a fiery tone.

Trump had just declined to debate Sanders, saying in a statement that it seemed "inappropriate that I would debate the second place finisher." Sanders challenged the presumptive Republican nominee saying he comes across as a "tough guy," and suggested he should prove it.

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"If you're so tough, let's sit down and have that debate," Sanders said to reporters in Los Angeles, where he was touring the Inglewood Oil Field.

Sanders still hoped for another change of heart.

"I heard that he was gonna debate and now you're telling me that he is not gonna debate me. Well, you know, I hope that he changes his mind again. Mr. Trump is known to change his mind many times in a day," Sanders said.

And he added that he thought Trump had had a "good idea" in proposing to give some of the profits to charity.

During a "Jimmy Kimmel Live" interview earlier this week, Trump had said he'd be willing to debate the Vermont senator.

After Sanders sent out a tweet declaring "Game on," however, Trump's campaign seemed to suggest that the presumptive GOP nominee was joking. But on Thursday, Trump told reporters in North Dakota that he was indeed interested in debating.

"I'd love to debate him...Maybe if we can raise for women's health issues or something-- if we can raise $10 or $15 million for charity," Trump said. "It should be in a big arena somewhere, and we can have a lot of fun with it."

Trump's reversal came less than an hour after Sanders' campaign announced the Vermont senator was ready to accept either of two television network offers to host the event.