David Letterman is pretty ticked off at Donald Trump, who implied that Barack Obama only got into Harvard because of affirmative action. The talk show host took Trump's friend Dr. Phil McGraw to task for the real estate mogul's comments about the president.

Trump has been hinting at a possible run for the presidency next year and, in addition to adding to the birther frenzy, has criticized Obama's presidency.

"It's all fun, it's all a circus, it's all a rodeo, until it starts to smack of racism. And then it's no longer fun," Letterman told McGraw last night on The Late Show, referencing Trump's comments about Obama's education.

McGraw attempted to defend Trump, saying, "I don't think he always thinks everything through. I think sometimes he's a little from the hip, and I don't think he has a racist bone in this body."

But Letterman made it clear that if Trump appeared on his show, he'd have to apologize. "If he comes back on this show, and I am not sure we want him back on this show under these circumstances, he ought to be prepared to apologize just for that kind of behavior."