(CNN) -

Mitt Romney says he's not running for president again. Mind's made up. No chance of changing it. Well, except for his "Dumb and Dumber" scenario.

Romney's reference to the Jim Carrey movie came up as he explained again on the Hugh Hewitt radio show Tuesday night all the reasons he's sitting out the 2016 presidential race.

Romney said, "someone else stands a better chance of winning than I do. Had that not been the case, had I believed I would actually be best positioned to beat Hillary Clinton, then I would be running."

Despite the calls from some Republicans for the 2012 nominee to take another shot in 2016, Romney again said, "I actually believe that someone new, someone not defined yet, someone who perhaps is from the next generation, will be able to catch fire potentially, build a movement and be able to beat Hillary Clinton.

"If I thought that weren't the case," he added, "then I would be running."

But Hewitt pressed him, saying that had to be subject to change if circumstances became different.

"I'm not going there," Romney responded, laughing. "This is something we gave a lot of thought to when I decided early on we're not going to run this time. I had the chance of running. I didn't win."

Then he added, "circumstances can change, but I'm just not going to let my head go there. I remember that great line from "Dumb and Dumber..."

As Hewitt said the line ("so you're telling me there's a chance"), Romney said, "there you go. You remember. So you're telling me I have a chance. Yes, one out of a million."

Romney then painted a scenario about as likely as Carrey's Lloyd Christmas winning over Lauren Holly.

"Well, let's say all the guys that were running all came together and said, 'hey, we decided we can't do it,'" he said. "You must do it. That's the one of a million we're thinking about."

Which allows the political media to say, so you're telling us there's a chance...