WASHINGTON – When asked what he would do if President Donald Trump challenged him based on his age or mental fitness in a debate, former Vice President Joe Biden said he would challenge the president to a push-up contest.

Biden made the comment during an interview with MSNBC's Mika Brzezinski that aired Monday on "Morning Joe." Brzezinski asked Biden about Democratic voters who wondered how he would handle someone like Trump, "who doesn't play fair," after he seemed unprepared for criticism from Sen. Kamala Harris in the Democratic debate in Miami.

"I realize that some have concluded that because I didn't respond very tough back to her, that how can I take on Trump? I've never had any trouble taking on anyone from Trump to Putin to Xi Jinping or anybody else," he said.

"I'm used to bullies," Biden added. He cited a line from former President Harry Truman, which he said his "grandpop" used to quote. When asked if he was going to "give 'em hell," Truman said, "No, I'm just going to tell them the truth. They're going to think it's hell."

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"That's all you've got to do with this guy," he said.

"I know it will be an ugly campaign. I know it will be not an honorable campaign that he'll run, but I'm not backing down at all from him," Biden said.

Brzezinski pressed Biden on what he would do if Trump "starts making fun of your age" or "your mental state."

"I'd say, 'Come on Donald. Come on man. How many push-ups you want to do here, pal? You know, I mean jokingly. You know, come on run with me man," Biden replied.

Biden told the story of a Trump supporter who called him "Sleep Joe" during a parade in Independence, Missouri.

"I said, 'Come run with me, Jack. Come on man,'" but the man declined, Biden said.

"I'm not going to get down in the dirt with him, that's the only place he knows how to fight. I'm just going to continue to talk about what he's doing why it so damaging to America," Biden added.

"I'm not going to take any guff from him."