COUNCIL BLUFFS, Iowa — Democratic presidential hopeful Bernie Sanders ratcheted up his criticism Tuesday of Republican front-runner Donald Trump, calling him a “coward billionaire” and mocking him for talking about Hillary Clinton’s trip to the restroom during the past presidential debate.

“He has discovered that women go to the bathroom, and it’s been very upsetting to him,” the senator from Vermont said of the real-estate mogul during a rally here that drew about 1,800 people. “This is a guy who wants to be president of the United States. He must have a very unusual relationship with women.”

Sanders was referring to Trump’s recounting Monday of Clinton’s late return to the Democratic debate stage following a commercial break Saturday night.

Republican presidential candidate Donald Trump remarked on Democratic presidential contender Hillary Clinton's brief absence from the debate stage on Dec. 19 saying, "Where did she go? I thought she quit." (Reuters)

"I know where she went — it's disgusting, I don't want to talk about it," Trump told a crowd in Grand Rapids, Mich. "No, it's too disgusting. Don't say it, it's disgusting."

Sanders shared with his audience that “I was there at that debate that happened Saturday night” in Manchester, N.H.

“Now I’ve got to be honest with you. I’ve got to be honest with you,” Sanders said, speaking in a mocking tone. “I also went to the bathroom. I know, I have to admit it. … This is the pathology. This is the guy who is leading in the Republican polls.”

Earlier in his hour-long remarks, Sanders also took aim at Trump for his hard-line position on immigration and his recent proposal to ban all Muslims temporarily from coming into the country. Sanders also lambasted Trump for his objection to raising the minimum wage and his advocacy of tax breaks for high-income earners.

“You try to divide the American people up, you try to get us to hate Mexicans, to hate Muslims,” Sanders said. “You keep the minimum wage low and you give huge tax breaks to millionaires. Mr. Trump, that is not what makes America great.”

Sanders’s appearance here came on the second day of a three-day swing through Iowa, the nation’s first caucus state. The state is key to Sanders’s fate. He has been leading in polls in New Hampshire, the second nominating state, but he has been trailing Clinton, former secretary of state, in Iowa.

Sanders plans to return to Iowa next week for another three-day swing.