Former Vermont Gov. Howard Dean (D) has endorsed Democratic presidential candidate Hillary Clinton, but he told The Washington Post in an interview published Tuesday that her rival, Sen. Bernie Sanders (I-VT), is a “phenomenon” who could pull out an upset.

“Bernie is a real phenomenon, not just a quirk of the polls. On the other hand, Hillary’s a pretty formidable candidate,” Dean told the Post in a Q&A. “I suspect Hillary is going to win this. She’s well qualified, she’s tough, and she’s been through all of this before. At the end of the day voters will choose a president who is basically ready for the job. I think that puts Hillary head and shoulders above everybody else in the race.”

The Post asked Dean, though, whether he could “dismiss the possibility that Sanders pulls off an upset.”

“No, I wouldn’t dismiss any possibility,” Dean said. “The voters end up making up their minds, not those of us who are pundits for a living.”

Dean, the former chairman of the Democratic National Committee, also told the Post that he disagreed with a rule the party had instituted regarding debates.

“The only thing I would disagree with is that I have heard that there’s a rule that says, if you participate in an unsanctioned debate, then you can’t participate in a sanctioned debate,” he told the Post. “That I don’t agree with. It’s not right.”

Read the full interview.