WASHINGTON (AP) -- The latest mystery in Washington espionage circles came to light in an unlikely venue: the floor of the U.S. Senate.

Tucked inside Congress' new blueprint for U.S. intelligence spending is a highly classified and expensive spy program that drew exceptional criticism from leading Democrats.

In an unusually public rebuke of a secret government project, Sen. Jay Rockefeller of West Virginia, the senior Democrat on the Senate Intelligence Committee, complained Wednesday that the program was "totally unjustified and very, very wasteful and dangerous to the national security." He called the program "stunningly expensive."

Rockefeller and three other Democratic senators -- Richard Durbin of Illinois, Carl Levin of Michigan and Ron Wyden of Oregon -- refused to sign the congressional compromise negotiated by others in the House and Senate that provides for future U.S. intelligence activities.

The compromise noted that the four senators believed the mystery program was unnecessary and its cost unjustified and that "they believe that the funds for this item should be expended on other intelligence programs that will make a surer and greater contribution to national security."

Each senator -- and more than two dozen current and former U.S. officials contacted by The Associated Press -- declined to further describe or identify the disputed program, citing its classified nature. Thirteen other senators on the Intelligence Committee and all their counterparts in the House approved the compromise.

Despite objections from some in the Senate, Congress has approved the program for the past two years, Rockefeller said.

The Senate voted to send the legislation to President Bush on Wednesday night. The bill is separate from the intelligence overhaul legislation that also won final congressional approval Wednesday.

The rare criticisms of a highly secretive project in such a public forum intrigued outside intelligence experts, who said the program was almost certainly a spy satellite system, perhaps with technology to destroy potential attackers. They cited tantalizing hints in Rockefeller's remarks, such as the program's enormous expense and its alleged danger to national security.

A U.S. panel in 2001 described American defense and spy satellites as frighteningly vulnerable, saying technology to launch attacks in space was widely available. The study, by a commission whose members included Donald H. Rumsfeld prior to his appointment as defense secretary for Bush, concluded that the United States was "an attractive candidate for a Space Pearl Harbor."

Sending even defensive satellite weapons into orbit could start an arms race in space, warned John Pike, a defense analyst with GlobalSecurity.org, who has studied anti-satellite weapons for more than three decades. Pike said other countries would inevitably demand proof that any weapons were only defensive.

"It would present just absolutely insurmountable verification problems because we are not going to let anybody look at our spy satellites," Pike said.

Rockefeller's description of the spy project as a "major funding acquisition program" suggests a price tag in the range of billions of dollars, intelligence experts said. But even expensive imagery or eavesdropping satellites, as long as they're unarmed, are rarely criticized as a danger to U.S. security, they noted.

"From the price, it's almost certainly a satellite program," said James Bamford, author of two books about the National Security Agency.

Another expert agreed. "It's hard to think of most any satellite program, at least the standard ones, as dangerous to national security," said Jeffrey T. Richelson, who wrote a highly regarded book about CIA technology in 2001.