Camden Fine, CEO of the industry group Independent Community Bankers of America, said Dimon is channeling his inner Mr. Potter, the big banker from the classic Frank Capra movie who sought to undermine the underdog loan provider George Bailey.
"I guess we found out yesterday that if you have a policy disagreement with Mr. Dimon, you're a jerk," Fine told CNBC's "Squawk Box" .
"Mr. Potter said something akin to that to George Bailey in 'It's a Wonderful Life' because Potter wanted to turn Bedford Falls into Potterville. I guess Jamie wants to turn America into Jamieville," Fine said.
A spokesperson for JPMorgan said the company did not immediately wish to comment.
The barbs began when Fine responded to an op-ed by Dimon, which called on large and small banks to unite. Fine said the article reflected big banks' attempt to pass their legislative agenda on the backs of small banks.
In response, Dimon called Fine a "jerk" during a "Squawk Box" interview on Wednesday.
On Thursday, Fine said large banks should push their agenda through Congress on the merits. He said the regulatory relief small and community banks are pursuing has broad bipartisan support, while too-big-to-fail banks face a tougher partisan divide.
He noted that less than 2 percent of the nation's banks control more than 70 percent of the country's financial assets. He said that creates an uneven system in which small banks are suffering the regulatory consequences of the Wall Street excess that led to the financial crisis.
"You have to understand from a community bank point of view that nearly every regulation under which we suffer was triggered by some egregious act or legal violation by a mega-bank," he said.
Dimon said Wednesday he was sympathetic to community banks and would like to see regulatory burdens on them reduced, regardless of whether large banks got relief.