Senators from Virginia and Maryland are urging lawmakers negotiating a Federal Aviation Administration reauthorization bill to omit provisions that would add long-distance flight slots to Ronald Reagan Washington National Airport, a controversial issue that threatened to hold up the bill in the past.

A perimeter rule Congress set in the 1960s establishes National as a short-haul airport, limited to flights within 1,250 miles, with some exemptions. In a letter Friday, Democratic Sens. Mark Warner and Tim Kaine of Virginia and Benjamin L. Cardin and Chris Van Hollen of Maryland argue that any additional flights would overburden the airport’s capacity.

The Senate Commerce, Science and Transportation Committee advanced its FAA reauthorization bill in February with an amendment that would add 10 total flight slots — five additional flights into DCA and five out — a major decrease from earlier proposals, such as one from Sen. Raphael Warnock, D-Ga., to add as many as 56 additional flight slots.

The House, however, voted down multiple proposals to add National flights in its FAA bill, which passed last July, 351-69.

The four senators indicated last summer that they intend to oppose the FAA bill if it includes any additional flight spots. They added that extra flights would siphon customers from Washington Dulles International Airport, which is located roughly 30 miles from downtown Washington. National is fewer than 5 miles away.

“The airport was designed to accommodate 15 million passengers, but this year, in part thanks to previous Congressional carve-outs to DCA’s slot and perimeter rules, it is on pace to serve 25 million,” they write in the letter. “The ten new flights, which would be allowed to fly beyond DCA’s 1,250-mile perimeter, would mostly likely be flown by larger aircraft, with a correspondingly larger strain on DCA’s resources.”

Advocates for more flights, like Senate Commerce ranking member Ted Cruz, R-Texas, contend the extra slots would meet the travel demands for residents of D.C. and its growing metropolitan area as well as tourists. It also would make for a faster commute for many members of Congress from outside of the current 1,250-mile perimeter.

The proposal is also causing a divide between air carriers, and prompting an increase in lobbying dollars. United Airlines, which opposes any flight additions, has led the pack in lobbying. Dulles is a United hub.

Delta Air Lines, on the other hand, joined a group dubbed the Capital Access Alliance that is advocating for additional slots.

The slot proposals are likely to be a point of debate as the House and Senate negotiate a final bill. Senate Commerce Chair Maria Cantwell, D-Wash., said this month that pre-conference negotiations on the bill are just ramping up.