Herman Cain -- Ex-Con Had My Back During Republican Debate

Herman Cain's rep was dead wrong when he told TMZ that Oksana Grigorieva's bodyguard only protected the Republican presidential candidate one time ... and we have the photo proof.

TMZ broke the story ... Kristian Otto Herzog -- who served jail time in 2009 for impersonating a police officer -- was a member of Cain's security detail during a campaign stop in Detroit on Oct. 21.

Cain's rep told us Herzog only worked for the campaign on that ONE occasion ... and insisted he would not be asked back to work any future Cain events.

But TMZ has obtained photos of Herzog working security at the Republican presidential debate in Las Vegas on Oct. 18 ... even taking a photo with Cain's arm draped around him at the hotel after the event.

Now, Cain's rep has changed his tune -- telling us he "did some research" and could now confirm Herzog did work the debate along with a 3rd event in Ohio on October 22.

Sources connected to the Cain camp tell us ... Herzog may have been able to slip through the screening process because the campaign was in a "rush" to find extra security and didn't have time to perform a more thorough background check.

TMZ spoke with Herzog ... who insists he was NOT fired ... but says he felt pressured to resign after we published our story this weekend.