CBS News executives were red- faced yesterday trying to explain how David Letterman used unaired news footage of Sen. John McCain with Katie Couric to embarrass the Republican presidential candidate.

McCain canceled his appearance on Letterman’s show late Wednesday, several hours before he was due to appear – claiming he had to return to Washington to deal with the financial crisis.

Get MORE TV News at NYPost.com

But when Letterman discovered the Senator sitting down with Couric at the same time he was supposed to be taping “Late Night,” he unloaded on McCain.

“I’m more than a little disappointed by this behavior,” Letterman told viewers. “This doesn’t smell right.”

“This is not the way a tested hero behaves. Somebody’s putting something in his Metamucil,” he said.

Later in the show, Letterman showed an internal, live video of McCain being tended to by a make-up artist before the Couric interview. Both Couric and Letterman are on CBS.

Letterman said on the air that McCain had called him personally to apologize and said he was racing to the airport.

“He doesn’t seem to be racing to the airport, does he?” Letterman told viewers.

“I feel like we’ve caught him getting a manicure,” Letterman quipped, as a make-up woman dabbed at McCain’s face.

Asked if CBS officials had a problem with Letterman using the internal news feed, a spokeswoman for “The Evening News” refused to address the issue.

But several CBS News executives – who asked not to be identified – said that the stunt did not go down well within the news division.

“If we had done something like that to him, someone around here would end up getting fired,” one said.

News officials found out Letterman was using the internal feed shortly after it showed up on an internal CBS feed carrying the “Late Show” taping.

“They were pretty aggravated,” a CBS News source told The Post.

“But they were not about to start a fight with Letterman,” the source said. “We’re in the middle of a heavy, heavy news cycle and Letterman is Letterman.

“He does whatever he wants and always has.”

McCain spokeswoman Nicole Wallace said that the campaign canceled on Letterman because it “felt this wasn’t a night for comedy.”