"Others have rightly pointed out that this situation raises the question of whether the agency should create receiver standards so that license holders operate only in their licensed spectrum," Kerry and Graham wrote.
But they said tougher receiver standards for GPS devices is a "long-term" issue that is of little help to LightSquared, which is reportedly near bankruptcy.
"In the short-term, we urge you to work with industry and the relevant federal agencies to find consensus on alternate spectrum for LightSquared's proposed network," they wrote. "Advancing LightSquared's network in a consensus manner would increase competition in the wireless broadband market and promote the public interest."
The senators did not suggest which block of spectrum the government should set aside for LightSquared. Spectrum, which powers all wireless devices, is a limited and valuable resource.
The letter is dated March 29 and was obtained by The Hill on Tuesday.
Some Republicans have questioned whether the administration showed inappropriate favoritism to LightSquared. Sen. Chuck Grassley (R-Iowa) has blocked President Obama's two nominees to the FCC in an effort to force the agency to release internal records on its review of the company.
Republicans on the House Energy and Commerce Committee have launched their own probe of the FCC's review of LightSquared. They question why the agency allowed LightSquared to get as far as it did in the regulatory process.
The White House and the FCC have denied giving any special treatment to LightSquared, but expanding high-speed Internet access is a priority for the administration.