Buoyed by their recent political convention and reports critical of US military readiness, Democrats on Capitol Hill are on the offensive in redirecting the Reagan administration's planned defense buildup.
Before Congress adjourned for the San Fransisco gathering that nominated Walter F. Mondale and Geraldine A. Ferraro, House and Senate were at an impasse over a Pentagon spending figure for 1985. Within hours after the House-Senate conference committee reconvened this week, however, the Republicans apparently blinked and agreed to a defense spending figure at least $2 billion lower than the $299 billion that the administration had been holding out for.
''I think we're in a much stronger position than we were,'' said Rep. Patricia Schroeder (D) of Colorado, a member of the House Armed Services Committee assigned to the conference group now working out differences between House and Senate versions of the defense authorization bill.
A newly revealed House Appropriations Committee report on deficiencies in the ability of US forces to sustain combat ''cuts our way,'' Congresswoman Schroeder said.
In recent months, the Pentagon has been subjected to continual hammering on the management of its record-breaking budget. Stories about high-priced spare parts, the disposal of such parts when they could be used elsewhere, and now warnings from military commanders that their armories are less than full have added to the public perception that the Defense Department could be better managed.
''I think even the Republicans in the House sense the feeling of the American people that there has to be some equalization in the defense budget, some change of priorities,'' said Rep. Joseph P. Addabbo (D) of New York, chairman of the House Appropriations subcommittee on defense.
On overall Pentagon spending for next year, the administration had already lowered its sights from 13 percent real growth (that is, not counting inflation) to 7.8 percent. The Democratically controlled House wants just 5 percent growth in defense spending, and the latest concession by congressional Republicans edges the likely final figure closer to that amount.
More significant may be actions by lawmakers to limit certain key weapons systems and the use of US forces abroad. Here, too, the politics of defense spending is evident.
On the MX strategic nuclear missile, for example, the administration originally asked for 40 missiles in the 1985 budget. The House chopped that number to 15, and even the Republican-dominated Senate just barely approved 21 missiles.
The MX, which is strongly opposed by Walter Mondale and the Democratic platform, is likely to be a key campaign issue. And some observers think the fervor and unity displayed in San Francisco could work against the MX, especially among those Democrats who have continued to favor the new missile.
''Maybe it's wishful thinking,'' Congressman Addabbo said Wednesday, ''but hopefully it will have convinced some of my Democratic colleagues to vote aganst the MX.''
In any case, even if some new missiles are approved, Democrats on the defense authorization conference committee are confident that they will be able to include a key provision tying continued production to progress on arms control and giving lawmakers another chance to vote on the MX.
On another important new weapon - antisatellite (ASAT) rockets fired from jet fighters - the administration also is likely to find itself influenced, if not limited, by congressional defense budget action.
Political maneuvering by Moscow - aimed at the presidential election as well as the congressional budget process - is also evident here.
Following its recent offer to talk with the US in Vienna about space weaponry , the Kremlin is trying very hard to force a delay in the US ASAT test planned for this fall. The Reagan administration, for its part, is doing everything possible to keep the hopes for the Vienna talks alive without actually agreeing beforehand not to test its new antisatellite weapon.
House defense spending conferees, led by Rep. George E. Brown Jr. (D) of California, are now pushing for a moratorium on such testing by the US Air Force as long as the Soviet Union continues its self-declared moratorium on ASAT flight tests. The Senate wants to allow ASAT testing as long as the administration can show it is seeking ''the strictest possible limitation'' on ASATs.
The likely outcome, according to those close to the negotiations on Capitol Hill, is that lawmakers will delay possible ASAT testing until 1985, when the dust from the November election (and the US-Soviet minuet over a meeting in Vienna) has settled.
''I'm very confident that we'll get something like that out of the conference ,'' a Democratic congressional source said.