TO judge from the outlines of its defense spending bill, the United States Senate may have slipped into a time warp where the cold war still chills and Pentagon dollars cascade.But the politics of arms procurement can't be quite so easily explained. The senators who voted to continue production of the B-2 and deploy a scaled-down version of the Strategic Defense Initiative (SDI) have a dilemma: how to fit "next generation" hardware into a shrinking Pentagon budget. No one questions the shrinkage. The administration announced a year ago a 25 percent reduction in forces by 1995, and the 1990 budget agreement mandates a steady contraction in defense spending. Waning military competition with the Soviet Union underlies these trends. The Gulf war, however, bolstered arguments for maintaining military strength. So which weapons can best meet US defense needs, at a price compatible with budget realities? The Stealth bomber's military potential was established over Baghdad by its smaller cousin, the F-117 Stealth fighter. But 15 B-2s are already in the pipeline - enough to carry out any special mission that might come their way in this post-cold-war era. At $864 million a copy, even the four additional bombers OK'd by the Senate mean far less money for other planes that have greater versatility. It's a bad choice. The Senate's decision to push SDI funding to $4.6 billion in fiscal year 1992 - less than the $5.2 billion asked for by President Bush, but considerably higher than the House's $3.5 billion - is also a poor choice. The "star wars," space-based aspects of SDI have rightly been shelved, permanently we hope. Proponents argue that the imminent possibility of rogue nations like Iraq lobbing missiles our way justifies quick deployment of smaller, land-based defenses against long-range missiles. But that possibility is not imminent. Long-range missiles - as distinct from shorter-range ones - are not sprouting all over the third world. Somewhere down the line, that threat could materialize, which argues for some continued research - and even more for diplomacy aimed at averting the spread of missiles. As it stands, the Senate bill would undo the Antiballistic Missile Treaty with the Soviets and work against further reductions in nuclear forces. To some extent, defense-minded senators are staking out bargaining positions in anticipation of the fall conference with the House. That exercise in compromise ought to favor the House's more moderate, more realistic approach to weapons purchases.