*Count David Letterman among the celebrities who are now linking Donald Trump’s recent comments about President Obama’s birth certificate and ivy league worthiness to flat out racism.

In a preview of tonight’s interview with Dr. Phil, Letterman offers his opinion on the “Celebrity Apprentice” host, saying the birther drama was amusing and all, but the questions about his academic record is another issue altogether.

“It’s all fun, it’s all a circus, it’s all a rodeo until it starts to smack of racism,” said Letterman. “And then it’s no longer fun.”

Letterman suggests that if Trump comes back to The Late Show — “and I’m not sure that we want him back” — the tycoon “ought to be prepared to apologize.”

Watch below. This broadcast airs tonight (April 29) on CBS.