DES MOINES — It’s a can’t-miss joke that Senator Bernie Sanders cannot stop telling.

“You know what Republicans believe? Let me quote that great American scientist Donald Trump,” Mr. Sanders says. “Trump believes that, of course, climate change is a hoax. And that it was created by the Chinese. Now this shocked me very much — not that he thought climate change was a hoax — I thought he would have thought it would have been created by the Mexicans or the Muslims.”

He told it a week ago in Fayette, the next day in Iowa Falls, two days after that in Mason City and then again in Davenport.

Each time, the crowd burst into laughter as Mr. Sanders, grinning, threw his hands in the air, seemingly exasperated.