A bipartisan group of lawmakers asked the Air Force this week not to split up a $40 billion contract for refueling tankers that is expected to be awarded by the end of the year.

Los Angeles defense contractor Northrop Grumman Corp. has teamed with European plane maker EADS to bid on the massive contract which could ultimately be worth more than $200 billion which would replace an Air Force aging tanker fleet with 179 new planes. Chicago-based Boeing Co. is Northrop and EADS’ main competitor for the job.

Fourteen U.S. senators and 52 congressmen signed letters this week to Air Force Secretary Michael Wynne arguing that awarding the contract to a single team would help keep the program under budget.

“We strenuously oppose any attempt to change the current acquisition strategy to facilitate a dual procurement or ‘split buy’ approach,” the senators wrote.

Though some analysts have said splitting the contract would encourage competition and keep down costs, lawmakers said coordinating two tanker programs would be difficult and costly.