The U.S. Air Force "should hold a full and open competition" to choose the best available aerial refueling tankers at the best price, according to an amendment inserted in the U.S. Senate's defense spending bill by Sen. Jeff Sessions, R-Mobile.

Sessions' amendment to the fiscal 2008 authorization bill is the latest step in a campaign to promote competition for one of the largest defense deals in a generation. The bill is expected to gain Senate approval as early as this week.

The tanker contest pits Boeing Co. against a team led by Northrop Grumman Corp. and EADS North America for an initial contract estimated at $40 billion. The winner-take-all deal for 179 new tankers is expected to be awarded by year's end.

Los Angeles-based Northrop would assemble its KC-30 tankers in Mobile, creating up to 1,500 direct jobs, if selected by the Air Force over Boeing's KC-767. Northrop supporters have labored to win political support for the KC-30, a plane that has been criticized by some in Congress because it is based on a French-made Airbus A330 commercial jet.

Northrop earlier this year threatened to drop out of the contest because of concerns that the Air Force's bidding criteria were skewed to favor Boeing, which boasts close ties to the service and built its existing fleet of more than 500 KC-135 tankers. In particular, a "Buy American" amendment proposed by the House of Representatives last year would have disqualified the KC-30 because of its foreign roots.

Boeing, which would assemble its tankers in Everett, Wash., has been named by military experts as the heavy favorite for the contract, in part because of its political influence.

Sessions' amendment, one of about 50 passed by the Senate late Monday, discourages the Air Force "from taking any actions that would limit the ability of either of the teams seeking the contract...from competing for that contract."

"I am pleased that my colleagues in the Senate unanimously agreed that the goal of this program should be to select the best aircraft through a robust and open competition that is focused on the capabilities of the airplane," Sessions said. "This amendment sends the message that merit, not politics, should be the basis for the selection."

Boeing supporters said the Air Force has made strong efforts to avoid showing any favoritism thus far.

"It appears that the Air Force is bending over backward to ensure a fair and open competition," said George Behan, a spokesman for U.S. Rep. Norm Dicks, D-Washington.

Speaking to reporters in Washington, D.C., Northrop executive Paul Meyer welcomed the amendment, saying that it would heighten confidence in the Air Force's selection process and thus make Congress more likely to fund the program.

At Tuesday's 90-minute media briefing, Meyer, who is Northrop's vice president for air mobility systems, and U.S. Rep. Jo Bonner, R-Mobile, took aim at sniping about the KC-30's pedigree.

Some 58 percent of the plane is currently "U.S. content," Meyer said, adding that foreign manufacturers play a significant role in the M-1 tank, the Joint Strike Fighter and other high-profile weapons programs. Even the engines that power the Air Force's existing fleet of KC-135s are 50 percent French-manufactured, he said.

Bonner touted the benefits of competition both to airmen and taxpayers.

"Clearly, today's defense market is global," he said.

A spokesman for Boeing said the Chicago-based company "supports full and open competition for this important program."

The Senate bill includes about $315 million in research and development funding for the Air Force tanker program next year, the full amount requested by the Bush administration.

The defense bill must still be approved by the Senate and then combined with the House version by a conference committee.