The pressure on the Federal Communications Commission from Democrats on the Hill who don't want TV stations in the "duplex gap" continued to mount Tuesday, two days before the FCC is scheduled to vote on a proposal to do just that.

In a letter to FCC Chairman Tom Wheeler, a quintet of powerful senators urged "extreme caution" if the FCC is planning to put full-power stations in the duplex gap, the buffer between wireless uplink and downlink spectrum where the FCC is planning to put unlicensed wireless devices including microphones used for TV station newsgathering, as well as sports events like the Super Bowl and theatrical productions.

Sens. Richard Blumenthal (D-Conn.), Patrick Leahy (D-Vt.), Charles Schumer (D-N.Y.), Ron Wyden (D-Ore.), and Cory Booker (D-N.J.) wrote the chairman that almost every stakeholder involved--broadcasters, public interest groups, wireless companies--are concerned about placing the TV stations in the gap (which also means in the wireless portion of the spectrum band.

"This proposal would deny millions of Americans the innovative promise of unlicensed spectrum and the economic benefit it brings," they said flatly. "Furthermore, it would inhibit live news coverage and emergency communications by leaving local broadcasters in some markets with no reserved spectrum for the wireless microphones that are essential to cover breaking news," they said.

The letter followed one from a trio of House Dems last week saying virtually the same thing. New York City mayor Bill De Blasio also weighed in with duplex-gap concerns.

The FCC is scheduled to vote on the proposal Thursday (Aug. 6), although it has already delayed the vote once -- from July 16 -- after the duplex gap critics began making a lot of noise.

FCC chairman Tom Wheeler proposed putting a handful of stations in the duplex gap to help the FCC free up as much contiguous wireless spectrum as possible. The senators agree with that goal, but not at the expense of wireless mics.

They said the FCC "must ensure adequate spectrum is available for unlicensed use on a contiguous nation-wide basis, and that local broadcasters can continue to report the news and serve their local communities without undue disruption.”

There is an alternative proposal before the FCC that would have it reserve a second channel in those duplex gap markets, which would free up an additional 6 MHz that could go toward a TV signal or wireless mic or other unlicensed device.

If the chairman wanted to entertain that alternative, he would likely have to put it out for comment.

A trio of House Democrats has asked the FCC not to put TV stations in the duplex gap, but instead make sure stations in all markets can use that spectrum for wireless mics.

The FCC is expected to vote on putting a handful of stations in the gap as part of an incentive auctions procedures public notice teed up for a vote at the Aug. 6 meeting. But the proposal has been getting a lot of pushback, which helped move a planned July 16 vote on the procedures notice into August.

In a letter to FCC Chairman Tom Wheeler dated July 28, Reps. Bobby Rush (D-Ill.), G.K. Butterfield (D-N.C.) and Tony Cárdenas (D-Calif.) said TV stations in their communities would be "severely impacted" by the FCC's proposed decision to put some stations in the duplex gap, which it had initially planned to reserve nationwide for mics and other unlicensed devices.

Putting some stations in the gap will potentially allow the FCC to reclaim more spectrum for wireless broadband by freeing up a larger swath of spectrum nationwide.

But broadcasters and wireless companies have pushed back because of potential interference to both stations and wireless operations--the duplex gap is the buffer between uplink and downlink operations in the wireless band.

The legislators pointed out that the FCC has already eliminated exclusive wireless mic channels as part of the incentive auction plan, proposing the duplex gap as a new home.  "Now we understand that the commission has reversed course and intends to move forward with a new proposal that will leave local broadcasters in some markets with no reserved spectrum for wireless mics that are essential to covering breaking news," they said. The mics are also used in sporting and theatrical events.

"We urge the commission to reject this proposal and instead preserve the duplex gap for use by wireless microphones in all markets," they told Wheeler.