The Senate refused to stop production of the multibillion-dollar B-2 Stealth bomber, the futuristic warplane designed to penetrate enemy radar undetected and deliver a nuclear payload.

Warned that the Soviet Union continues to build its defense system despite the breakdown of its economic system, the Senate on Monday rejected 50-44 a proposal that opponents charged would kill the B-2 program.Offered by Sen. Patrick Leahy, D-Vt., the proposal would have stopped the program after purchase of six planes, none in the current fiscal year, for flight testing. The Pentagon has envisioned a fleet of 70 B-2s at $65 billion.

The B-2 showdown highlighted the Senate's passage, on a 79-16 vote, of the fiscal 1991 defense appropriations bill, representing the largest single-year reduction in military spending since the Vietnam War.

The $268.2 billion appropriations measure represents $20 billion less than what President Bush asked for at the beginning of the year, and nearly $16 billion below what Congress approved last year.

Left in the bill by rejection of the Leahy amendment was about $5 billion for the acquisition of two B-2s this year, plus components for six more that would be purchased next year. The House voted to cancel the B-2 program, allocating $1.6 billion to complete testing. The difference will have to be worked out by a conference committee.

Defending the high-tech bomber, Sen. Sam Nunn, D-Ga., said, "We're talking about revolutionary technology, a revolutionary plane that is going to change the nature of warfare and change it in our favor."

Opponents argued against the B-2 program, citing cost and the lack of need for another bomber with the Cold War at an end and the Soviet economy coming apart.

"This is a glaring example of misplaced priority," Sen. Timothy Wirth, D-Colo., said. "We do not need or can afford a new penetrating bomber."

The Senate also rejected, 50-46, an amendment by Sen. Kent Conrad, D-N.D., that would have removed 30,000 troops - in addition to the 50,000 in the bill - from Europe, but by voice vote adopted an amendment cutting the 50,000-strong U.S. force in Japan by 10,000 a year unless Japan bears the cost of keeping them there.

The bill included $78.5 billion for military personnel, a cut of $500 million from the administration's initial request, that would provide for a reduction of 77,100 in manpower.