In the letter, the lawmakers said those layoffs would affect hundreds of safety and maintenance workers and thus "would put the flying public at risk."
A reduction in funds also could delay the implementation of the Next Generation Air Transportation System (NextGen), which the Democratic senators say "will reduce total flight delays by about 21 percent while providing $22 billion in cumulative benefits to the traveling public, aircraft operators and the FAA."
NextGen would upgrade the ground-based World War II-era air traffic control system to a satellite-based system that can more accurately track airplanes on the ground and in the air.
Delays cost passengers $16.7 billion in 2007, according to the National Center of Excellence for Aviation Operations Research.
The FAA bill expired in 2007 and has gone through 17 short-term extensions.
Senate leaders have said they intend to move the bill quickly, with the intention of passing the measure the week of Feb. 14.