Senate Democrats anticipate a long week of debate on the Federal Aviation Administration reauthorization bill, which is set to come to the floor Monday.

But as of Friday afternoon, negotiations were still ongoing as lawmakers tried to iron out their differences about how to modernize the massive national aviation system.

An aide to Senate Majority Leader Harry Reid (D-Nev.) said Democrats expect Republicans to stall the FAA bill. In recent weeks, Democrats have increased their attacks on Republicans for obstructing the progress of legislation in general.

But the GOP doesn’t have a piece of legislation to target right now as Democrats hadn’t produced a bill as of Friday.

A spokesman for the Senate Commerce, Science, and Transportation Committee said talks with the Finance Committee are still ongoing. The spokesman said the two committees are trying to secure funding for modernization of the airline system, which could include increased taxes or fees.

A proposal submitted by the Commerce Committee would include a $25 surcharge per flight, while the Finance Committee seeks an aviation field tax. Either proposal could generate about $400 million to modernize aviation technology from a ground-based to a satellite system, according to a Democratic committee aide.

The Congressional Budget Office estimates that the FAA modernization plan would cost $50 million. Some of the other programs in the bill include improving air-cargo infrastructure, safety training for pilots and developing eco-friendly technology.

Lawmakers have come under increased pressure to upgrade the aviation system, especially after two airline jets nearly crashed into each other at Baltimore-Washington International Airport in January. Increasing the likelihood that Congress will act, thousands of flights were grounded in early April after it was revealed the FAA had been lax in performing maintenance inspections.