Many investors are growing concerned, MoffettNathanson Research’s Craig Moffett says today. The vehemence of the opposition to the $45B stock deal “seems to be spooking investors, who are for the first time taking seriously the idea that yes, even a deal that doesn’t reduce the number of competitors in any one market could be found to be either anticompetitive (by the DOJ) or not in the public interest (FCC),” he says. Yesterday, the FCC stopped its informal clock on the deal for a second time, to deal with programmer concerns that their deal terms will be made public. (Comcast says most big transactions run into delays.) Meanwhile, FCC chairman Tom Wheeler has been talking tough about the need to protect net neutrality, which could result in several demands on Comcast as well as new regulations.

Moffett has an imaginative way to prove his thesis. He checked the spread between the trading prices for Time Warner Cable and for Comcast. Since the cable giant is offering 2.875 of its shares for each TWC one, they should trade pretty much in sync with each other if investors are confident that the deal will close.

Comcast-TWC stock spreadBut he found that the spread has been increasing lately (see chart).

By his calculations, the Street gives the deal a 75% chance of approval, at best. “There’s some irony here,” he adds. “Initially, investors didn’t like this deal. Now the market loves the optionality of wireless, Enterprise and synergies, and the share buybacks that will follow. Now they really, really want the deal to close.”