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"And this is on top of this birther issue," he said of GOP questions about Obama's birthplace throughout his presidency, "which we heard from Donald Trump and others, a racist effort to try to delegitimize the president of the United States.

"Can you imagine that? To say that, well, he is not really the president, he wasn't born in the United States, which is nonsense."

"You know it's a funny thing, on that issue, my dad, as I mentioned, came from Poland. I'm running for president. Guess what? Nobody has asked for my birth certificate," he added to cheers from the South Carolina audience.

"Maybe it's the color of my skin. I don't know."

Sanders was then asked if he thought that Trump, the GOP presidential front-runner, questioned Obama's citizenship because he's a racist.

"I'm not a psychoanalyst, and boy would a psychoanalyst have an interesting time with Donald Trump. But ... there was an effort to delegitimize the president. Look, you can disagree with Obama all you want, but to say that the president of the United States, who won an election fair and square, was not a legitimate president really undermines what we are as a nation."

Sanders added that he "absolutely" thought that racism played a role for some Republicans who questioned Obama's birthplace.