Donald Trump thinks Florida Sen. Marco Rubio has a perspiration problem that could impact how seriously he'd be taken by Russian President Vladimir Putin if he makes it to the White House.

"I always say, I have never seen a young person sweat like Marco Rubio," the Republican front-runner said during a stump speech in Southern Iowa Wednesday. "He drinks more water — he's like a machine. Water, water, water. Sweats, gives a speech."

"Think of Putin," Trump told the boisterous crowd. "Pretty tough cookie, right?"

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He continued, "I think [of] Rubio and I'm saying, you have to be cool. You have to be really cool. And Rubio's going to meet him and walk in, and he's sweating — sweat is pouring down."

"And Putin's going to look at him and say, 'What the hell is wrong with this guy?'" Trump said.