Pai said this week that the conditions, including spinning off Boost Mobile, holding down prices and pledging 5G buildouts, made the deal in the public interest and he planned to circulate an order approving it, which his fellow Republican commissioners, comprising a majority, indicated they would support.
In the letter, the senators, who opposed the deal as previously constituted, said they still oppose it and asked both the FCC and Justice Department to reject it.
They cited the behavioral conditions, but made no mention of the Boost spin-off.
They said the conditions are mostly unrelated to merger-specific harms and thus the type of condition the chairman has criticized in the past. They also said they were worried about the process and that the conditions appeared without public input or "visibility."
As a result, they said, the FCC should give the public 30 days to weigh in on the conditions, though they did not need that long to say they don't think they make the deal palatable.
Signing on to the letter were Sens. Tom Udall (D-N.M.), Amy Klobuchar (D-Minn.), Richard Blumenthal (D-Conn.), Elizabeth Warren and Ed Markey (both D-Mass.) and Cory Booker (D-N.J.).