Potential 2012 presidential candidate Donald Trump lashed out at comedian Jerry Seinfeld on Wednesday when the actor backed out of Trump’s son’s charity event.

According to the New York Post, Seinfeld scrapped a September appearance at Eric Trump’s St. Jude Children’s Research Hospital benefit because he disagrees with Trump’s debate on President Barack Obama’s citizenship.

“I just learned you canceled a show for my son’s charity,” Trump wrote in a letter to Seinfeld, “because of the fact that I am being very aggressive with respect to President Obama, who is doing an absolutely terrible job as our leader.”

“The Apprentice” reality television star said he regretted partaking in Seinfeld’s program flop, “The Marriage Ref.”

“We don’t care that you broke your commitment,” Trump wrote, “even though the children of St. Jude are very disappointed, and despite the fact that your manager clearly stated you are ‘truly a man of his word.’ What I do feel badly about is that I agreed to do, and did, your failed show, ‘The Marriage Ref,’ even though I thought it was absolutely terrible … Despite its poor ratings, I didn’t cancel on you like you canceled on my son and St. Jude. I only wish I did.”

Trump ended the letter with this message, “You should be ashamed of yourself!”

Though Seinfeld committed to the foundation event appearance in January, the comedian’s rep says “Jerry has grown increasingly uncomfortable” with businessman Trump’s questioning of Obama’s birth certificate.

Seinfeld isn’t the first celebrity to have made waves with Trump. In 2006, the real-estate powerhouse called comedienne Rosie O’Donnell a “loser” after she said he was a “snake-oil salesman.”

“Rosie’s a loser. A real loser,” Trump told People magazine. “I look forward to taking lots of money from my nice fat little Rosie.”