Paul Sancya/AP Photo

Rick Perry delivered his biggest fumble of the campaign to date when he failed to name the third agency he would eliminate if he were to become president during a Republican presidential debate in Rochester, Mich.

“It’s three agencies of government when I get there that are gone – Commerce, Education and the um, what’s the third one there? Let’s see. Oh five – Commerce, Education and the um, um,” Perry said.

Mitt Romney, standing two podiums to Perry’s right, offered the Environmental Protection Agency as a suggestion.

“EPA, there you go,” Perry said.

But then, the Texas governor quickly retracted his statement, saying the EPA doesn’t need to be eliminated but simply rebuilt.

Again, he tried to name the third mystery agency.

“But you can’t name the third one?” CNBC moderator John Harwood asked.

“The third agency of government I would do away with - the education, the uh, the commerce and let’s see. I can’t the third one. I can’t. Sorry Oops.”

The third agency Perry couldn’t think of was the Department of Energy, which he rails against on the stump nearly every day.

Perry finally remembered the third agency 15 minutes later after referring to his notes, saying “By the way, it was the Department of Energy I was talking about.”

Just minutes after the conclusion of the debate, Perry took to the spin room and told reporters he was embarrassed by his gaffe.

“Speaking of boots, I’m glad I had my boots on tonight because I sure stepped in it out there,” Perry joked. “I stepped in it. Man, yeah it was embarrassing. Of course it was.

“From time to time, you may forget about an agency that you are gonna zero out,” he said. “Everybody tomorrow will understand the Energy Department is one of those that needs to be done away with.”

Advisers to Perry characterized his slip up as a “human moment,” saying it was a “stumble of style not of substance.”

Rep. Michele Bachmann expressed sympathy for the Texas governor has he struggled to remember the name of the agency.

“It was a tough moment,” she said on CNBC after the debate. “All of us recognize it was something none of us would want to go through. I felt bad for him.”

Eric Fehrnstrom, asenior advisor to Mitt Romney said, “There’s nothing I could say that could darken the night Rick Perry has had.”

Sources told ABC News Romney’s green room sat in “quiet amazement” as the governor failed to answer the question.

Perry has received criticism throughout this campaign cycle for his poor debate skills, but his brain freeze Wednesday night may have been the most severe of his campaign stumbles, especially as the Texas governor has centered his job creation plan on unleashing the power of the energy industry.

Perry has previously said he believes there are too many debates this primary season, but on Wednesday night he said he would not back down from the upcoming debates.

“Absolutely not, I’ll be in South Carolina on Saturday and, hopefully, I’ll remember the Energy Department.”

Perry will continue engaging in damage control of the incident Thursday morning when he plans to appear on the major morning shows.