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Bernie Sanders.
Asked by CNN's Chris Cuomo whether Trump's aggressive push to persuade President Barack Obama to release his birth certificate demonstrated a racial bias, Sanders demurred.
But the Vermont senator nevertheless took a slight jab at Trump.
"I'm not a psychoanalyst, and boy would a psychoanalyst have an interesting time with Donald Trump," Sanders said, as the audience applauded.
Sanders told Cuomo that, although he could not speak to Trump's motivations, he believed the "birther" movement, which was driven by a doubts over whether Obama was born in America, was "a racist effort to try to delegitimize the president of the United States."
"It's a funny thing on that issue," Sanders said. "My dad, as I mentioned, came from Poland. I'm running for president. Nobody has asked for my birth certificate. maybe it's the color of my skin, I don't know."
He added: "I'm not going to speak to Trump, but do I think in some parts of that Republican base there is race involved, yes I do."
While Trump said that he is not willing to talk about whether he believes Obama was born in the US, he has hinted that he maintained his skepticism. Trump was once one of the most prominent people in the US questioning the authenticity of Obama's birth certificate.
"I really don't know," Trump said when asked about Obama's birth place last year.