Another fight has erupted between Mayor Bloomberg and Bill Thompson over grades — but this time, it’s personal and not about schools.

Bloomberg’s campaign yesterday began airing a TV commercial questioning the “D-minus” grade rival Thompson gave the mayor in their final debate Tuesday night.

“It was a major gaffe,” said Bradley Tusk, Bloomberg’s campaign manager.

Thompson prefaced his grade by saying, “I think I’ll be kind.”

But Bloomberg — leading by double digits in the polls — took the opposite tack. He described his Democratic opponent as a very able comptroller.

Even some of Thompson’s allies said they thought he should have been more generous.

“I think personally he deserved a C-minus,” said City Councilman Lew Fidler (D-Brooklyn), one of Bloomberg’s most outspoken critics and a Thompson backer.

Thompson spokesman Mike Murphy said the comptroller — who has accused the administration of grade inflation in the school system — was sticking with his original assessment.

“Bill just couldn’t give him a higher grade — you lose credit when you hijack democracy,” said Murphy, referring to the term-limits extension Bloomberg pushed through the City Council.

Bloomberg’s ad, titled “Really?” compares Thompson’s grade with those of two major newspapers that have endorsed the mayor.