Story highlights Bernie Sanders derisively called Donald Trump "Mr. Macho" for backing out of a one-on-one debate

Sanders has hunkered down in California, the site of his last big match-up with Hillary Clinton

Washington (CNN) Bernie Sanders has a sarcastic new nickname for Donald Trump: "Mr. Macho."

The Vermont senator, who is hunkering down in California for his last showdown with Hillary Clinton in the Democratic presidential race, used the nickname as he bashed the presumptive Republican nominee for embracing -- and then backing away from -- the idea of a debate against Sanders.

"Let me not worry about Hillary Clinton right now. Let me worry about Donald Trump -- this big, brave, macho guy, my goodness," Sanders told supporters in Oakland on Monday. "He said he wanted to debate Bernie Sanders, then he said he didn't, and then he said he did, and then he said he didn't."

"So I say to Mr. Macho," he said, "that I am open to a debate anytime, any place here in the state of California, hopefully before the June 7 primary."

Trump's bestowing of nicknames on opponents drew waves of attention during the primary season and helped him emerge atop the Republican field. He routinely mentions "Crooked Hillary" about his likely general election opponent, Hillary Clinton. Trump's moniker for then-GOP rival Ted Cruz, "Lyin' Ted," helped define the Texas senator negatively.

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