Nonetheless, Reed and others said they are baffled at how the administration could be proceeding with deployment while hesitating to test the system.
"If you're not confident enough to take a chance on a test, how can you say that this can engage successfully in a real operational mission?" the senator said in an interview.
Sen. Jack Reed (D-R.I.) said: "This has been a program so fraught with political calculation . . . that I would assume there's some political aspect to the delay."
Since the start of intercept tests in 1999, the Pentagon has had difficulty keeping up a consistent pace of flights. A couple of early test failures led to months of delay as the causes of the failures were probed.
Several successful intercepts after President Bush was elected gave the test program momentum. But after a failure in December 2002, MDA officials ordered a halt to more intercept tests until a newly designed booster could be completed.
The booster carries a "kill vehicle" into space. That vehicle -- a 120-pound package of sensors, computers and thrusters -- then separates and homes in on an enemy warhead, destroying it with a collision.
Development of the booster has proven more problematic than expected. As a result, several intercept tests that the Pentagon had planned to run last year and this year have been postponed, cancelled or recast as component tests.
The administration thus finds itself proceeding with deployment after only eight intercept tests -- all held before Bush's decision 21 months ago to start fielding a system in 2004. Five were hits, but all occurred under heavily scripted conditions.
All also involved a surrogate booster that flew only half as fast as the booster that will be used in the system. That booster has been launched successfully several times, but it has never flown attached to an actual kill vehicle. The maiden flight of the booster and kill vehicle is slated to be a central feature of the next test.
The absence of realistic flight testing has prompted Thomas Christie, the Pentagon's chief weapons evaluator, to say he will not be able to provide a confident assessment of the system's viability ahead of the planned deployment. In recent weeks, his Operational Test and Evaluation office has argued with MDA over widely different estimates of the system's likely effectiveness.
The differences, Obering said, reflect disagreement over which test data to include in computing the estimates.