Washington — Less than half a year ago Congress moved boldly against the MX missile, delaying money for testing and basing the new nuclear weapon. It was a major step for the lawmakers, who almost never reject a major weapon. And it appeared to be a message coming straight from voters weary of defense spending increases.
But that was last December.
This week, in one-two order, the House and Senate voted by a comfortable margin to release $625 million for deployment of the MX. The missile faces more tests on future spending bills, but the MX this week won impressive victories.
Not only did its traditional pro-defense allies back it, but so did a group of liberal, pro-arms-control Democrats. Even some congressmen, who earlier this month had argued for a nuclear freeze resolution, took the House floor on behalf of the MX.
The sudden shift of wind has taken some on Capitol Hill by surprise. In the Senate, which approved funds for the MX Wednesday by a 59-to-39 vote margin, a GOP leadership aide said it was ''fascinating to see the change.''
In interviews, participants on both sides of the MX issue cited the reasons. Among them, the main factor was one of the best lobbying efforts yet conducted by the Reagan White House. A steady, bipartisan stream of congressmen went to the White House for high-level briefings and for talks with the President about arms control as well as the arms buildup.
''The President in negotiating with Democrats was rather forthcoming,'' says Rep. Les Aspin of Wisconsin, a Democrat who helped lead the nuclear freeze fight but became a chief proponent of the MX. ''He gave a lot of assurances,'' says Representative Aspin, especially in the area of arms control.
President Reagan's influence on Capitol Hill appeared recently to be waning on budget matters. His uncompromising efforts to win a 10 percent after-inflation increase for defense failed, and he has stirred up partisan bitterness on domestic issues. But this week's success on the MX provides convincing evidence that the President can still round up bipartisan support when he needs it.
''I think mainly it's the President'' who caused the switch for MX, says Sen. Carl Levin (D) of Michigan, a foe of the missile, who also credits the bipartisan study commission headed by retired Air Force Lt. Gen. Brent Scowcroft for changing attitudes in Congress.
The Scowcroft Commission, which provided a balanced look at the US strategic position, gave what some have called ''credibility'' to the MX. The group proposed putting 100 new missiles into existing Minuteman silos in Nebraska and Wyoming. Although most experts agree this basing system would be vulnerable to Soviet attack, it is considered more practical than the Reagan plan proposed last November to put the missiles in a tight formation known as ''dense pack.''
Probably more important for winning over Democrats, the Scowcroft report ties arms strategy to arms control. It acknowledges that weapons such as the MX are destabilizing for world peace. Each MX would have 10 warheads, making it an inviting target for an enemy to strike first.
The commission cited the need for a smaller, single-warhead missile known as the Midgetman that would be more difficult to find and less valuable to destroy for the Soviets. Thus, the reasoning goes, the likelihood of nuclear war would be less. Since Mr. Reagan has endorsed the commission report, some newly converted MX supporters are voting for the missile as part of the total Scowcroft package, including the Midgetman.
Sen. Sam Nunn (D) of Georgia, a leading expert on defense, switched to the pro-MX side in the Senate vote Wednesday, despite doubts. ''I still don't like it,'' he says of the basing plan now proposed, and he concedes that building the giant MX is inconsistent with moving to smaller nuclear weapons. His best hope, Senator Nunn says, is that with the MX deployment, ''the Soviets will begin to be willing to go to single-warhead missiles.''
In fact, there is no guarantee that even the US will go to the smaller missiles. Senator Levin says Congress is buying a certain missile and uncertain arms control - ''the MX in the poke now and a pig in the poke later.''
''If the President backs off on that [the Midgetman], we can back off on the MX,'' counters Aspin. Meanwhile, he argues that voting for the missile is good for both nuclear disarmament and for Democrats.
Says the Wisconsin congressman, ''If the Democrats vote for the Scowcroft package, then we've done everything that we could to get [President Reagan] an arms control agreement, and we can certainly hold him accountable in November of '84.''
The vote in both houses this week is also a sign that Congress prefers to stand behind the President on defense strategy.