Democrats, in an attempt to sidestep an expected Republican filibuster in the Senate, have mapped out a possible House-Senate compromise on military spending even before the Senate begins formal consideration of the huge defense authorization bill.

For two months, Republicans have been holding the $300 billion defense measure hostage in a dispute over a Democratic proposal to require specific congressional approval for any testing in the president's space-based antimissile Strategic Defense Initiative that goes beyond the traditional, narrow interpretation of the 1972 Anti-Ballistic Missile (ABM) treaty.

But, in the meantime, Sen. Sam Nunn (D-Ga.) and Rep. Les Aspin (D-Wis.), chairmen of the Senate and House armed services committees, are nearing agreement on most aspects of the measure in an informal conference that is unusual if not unprecedented in the annals of Capitol Hill defense policy struggles.

According to Aspin, Democratic committee members, working with Republicans from the House but not the Senate, have reached tentative agreement on almost all but the most contentious big-ticket items such as the MX and Midgetman missiles and other arms control-related issues.

There is enough agreement on basic features of the bill to give guidance to appropriations committees if they must draft a spending measure for next year without benefit of a formally approved authorization bill, Aspin said. "I wouldn't say we've got it all done, but it's pretty close," he said.

The House version of the defense authorization bill passed May 20.

While House and Senate negotiators often have begun dickering over details of bills before they have passed both houses, lawmakers cannot remember going this far on such a major bill when it has passed only one house. But Democrats, frustrated by Senate GOP stalling tactics on issues ranging from arms control to campaign financing reform, contend this may be the only way to legislate for the Pentagon under the circumstances. Moreover, some suggest, it puts pressure on Senate Republicans to end their stall, let the bill pass and come to the bargaining table.

Normal congressional procedure calls for passage of authorization bills to set policy, followed by appropriations bills to allocate money based on those policy decisions. But budget pressures, coupled with major policy disputes such as the fight over the ABM treaty in the Senate, have clogged the authorization process, forcing more and more policy decisions to be made in appropriations bills. The Pentagon generally prefers to have critical policy decisions determined by the relatively conservative armed services committees rather than in the more hectic atmosphere of the appropriations process -- especially when the appropriations process breaks down and everything gets thrown into a huge catchall "continuing resolution" to fund all government operations.

Sen. John W. Warner (R-Va.), ranking Republican on the Senate panel, is caught in the middle and clearly uncomfortable, pushing for a way to resolve the Senate impasse without retreating on the ABM issue. Asked last week about the Nunn-Aspin negotiations, he said the Democrats are "depriving the Senate of a chance to work its will." Asked if the same cannot be said of the filibustering Republicans, he laughed and said, "I don't exactly come to this with clean hands."