“Sinclair is setting up front groups to run these stations to get around these rules,” he said.

Sinclair said it was “shocked and disappointed” by Mr. Pai’s decision, adding that it had informed the agency of its relationships with the buyers in the proposed divestitures. “At no time have we misled the F.C.C. in any manner whatsoever with respect to the relationships,” the company said.

Tribune declined to comment.

Mr. Aaron and other close observers of the Sinclair-Tribune deal expressed bewilderment over Mr. Pai’s latest move. Since becoming chairman in January 2017, Mr. Pai has enacted or proposed a wish list of policy changes advocated by Sinclair. The F.C.C. has eased a cap on how many stations a broadcaster can own and has relaxed a restriction on television stations’ sharing of advertising revenue and other resources.

“This is very surprising given Ajit Pai has used most of his tenure at the F.C.C. to do favors to benefit Sinclair,” Mr. Aaron said. “I’m actually having to pick up my jaw off the floor.”

Mr. Pai has asked the agency’s four commissioners to hand off its review of the merger to an administrative law judge to determine the legality of Sinclair’s proposal.

Jessica Rosenworcel, one of the commissioners, said she would vote to put the decision in the hands of a judge. Another commissioner, Michael O’Rielly, approved the order as long as a timeline for the hearing is defined. Three of the four commissioners have to approve the order, but it can’t proceed until all four have voted.

Mr. Pai has been the subject of an investigation by the office of the F.C.C.’s inspector general, which has been looking into whether he and his aides improperly pushed for the rule changes and whether they had timed them to benefit Sinclair. The status of the investigation is unclear.