CARLSBAD, Calif. — The country's biggest banks are "one major misstep" away from being broken up, the head of Zions Bancorp. (ZION) said Wednesday.

The debate over the largest banks' size and complexity resurged last week, when Attorney General Eric Holder said that some banks are too big to prosecute for financial crimes. His remarks reverberated in the Senate and added new fuel to the long-simmering movement to break up some banks. Now that movement is close to boiling over, Zions Chairman and Chief Executive Harris Simmons said.

"We're one major misstep away from that," Simmons told an audience at American Banker's annual Best Practices in Retail Banking Symposium. "Right now it's a parlor game. … Whether this gets enough momentum, who knows, but I think we have about one more major problem in large banks, and it's probably a done deal."

The debate has become so prominent in the industry that even more mainstream political figures are participating in it, the regional bank chief added.

"It's really interesting when you see people like George Will and Peggy Noonan writing about it," Simmons said, referring to the two Republican columnists. "This isn't just Sheila Bair speaking."

But Simmons is not advocating for a big bank breakup.

"Should the largest banks be broken up? I tend to think not," he said in an interview after his speech. "I tend to believe that the economy is really well-served by having banks — a lot of community banks, by having regionals, by having the large, national-in-scope kinds of institutions that we have. So no, I don't think that's the answer."

Zions has $50 billion in assets and a holding company that oversees eight banking units. Simmons said during his speech that he thinks this structure gives Zions a reputational and business advantage, compared with larger companies that have seen their reputations plummet since the financial crisis.

"It's no accident that the community-banking community has been much more effective politically. … If we are to rebuild this industry's reputation, it can only be done at the local level," Simmons said. "I for one don't believe it's beyond the ability of very large banks to play that role … but it does become more challenging with size."