When the US House of Representatives this week voted down all funds for constructing the MX missile, it did more than threaten a major new weapon. The missile funding could survive a tough test expected next week in the Senate.
But the 276-to-145 House vote on Tuesday shocked many on Capitol Hill by its wide margin. In the same Congress where only two years ago Republicans were labeled as ''Reagan robots'' and many Democrats feared bucking the White House, 50 Republicans broke ranks and voted against the MX.
The vote is ''a forerunner of things to come in the next Congress,'' says Rep. Joseph P. Addabbo (D) of New York, chairman of the House defense appropriations subcommittee, who led the fight against the MX.
In an area where members traditionally allow the administration to lead the way, Congress is serving notice that it is not going to follow blindly. And when the new Congress arrives next January with 26 additional Democrats in the House, they will almost certainly be even more difficult to control.
''It is such a symbolic vote,'' says Rep. Leon E. Panetta (D) of California, echoing the view on both sides of the issue. ''If we can knock this one down,'' he says, other so-called sacred cows of the military, from the F-18 fighter plane to the B-1 bomber and the M-1 tank, could be in jeopardy during the new Congress.
''We've just funded them because we were afraid to raise the question,'' says the Californian, who like most of his colleagues supports spending money to develop, but not build, the advanced MX missile.
The vote Tuesday would cut off $988 million for building the MX during 1983 but would not remove money for research and development for the system. It is the first time in memory that either house has voted against a nuclear weapon.
The MX vote came at perhaps the worst time for any controversial military hardware. The Congress is feeling keenly the message it received during the elections from Americans who have turned from hawks to doves since 1980. Even hard-line conservatives are conceding that the American public wants to cut defense.
At the same time, the concern about the threat of nuclear war has fed the national nuclear freeze movement.
''I think the American people deeply want to have the issue of arms control resolved with the Soviet Union,'' Panetta says.
''That's where the push ought to be,'' he says. ''Instead, it's how many more arms and missiles can we get to scare them (the Russians).''
Rep. Geraldine Ferraro (D) of New York has twice sided with the MX in earlier funding votes. This time she switched. ''I'm very much for the nuclear freeze, '' she says.
The final blow to the MX in the House was the ''dense pack'' basing system proposed by the White House. It would put the missiles close to each other in hardened silos in Wyoming. During floor debate, which happened to fall on Dec. 7, the anniversary of the attack on Pearl Harbor, dense pack was compared with the American planes lined up at the Hawaii air base so that the Japanese bombers could easily destroy them.
''Dense pack is not flying,'' conceded Rep. Jack Edwards of Alabama, the ranking Republican on the House defense appropriations panel. He added after the vote that if the MX ''does not go through the Senate any better than the House,'' then the Pentagon will have to rethink its strategy.
''They need to sit down and think on that third leg of the triad,'' he said, referring to the current Pentagon strategy that calls for nuclear missiles on land, sea, and in the air. The MX system is aimed to modernize the land-based weapons, which are seen as the weak link in American defenses.
''I still believe in the triad,'' said Representative Edwards. ''But I know when I've been licked.''
Senate majority leader Howard H. Baker Jr. (R) of Tennessee predicted that his members were ''somewhat more supportive'' of building the MX, although ''not necessarily'' the plan for basing the missiles.
However, a chief opponent, Sen. Ernest F. Hollings, claims to have 53 to 54 votes, about 40 of them solid, for cutting out construction money for the system. ''Members are looking for things to cut back in the budget, and they're skeptical of this basing system,'' says a spokesman for the South Carolina Democrat.
All sides say the Senate action will be close. It will also be difficult for members who are wary of both the dense-pack plan and of weakening the nation's defenses.
No one has a tougher decision than Wyoming Sen. Alan K. Simpson (R), whose state is designated to get the weapon and who must go home to an Episcopal church where many members and his bishop support the nuclear freeze.
''The root of all the nuclear activity is fear, whether it's a nuclear reactor or the MX,'' he says. ''I have to deal with that like anybody else.''
''What do we do? Reduce arms?'' he says, following the command of ''turning our swords into plowshares?''
''Part of me deeply wants to do that,'' he says, but he adds, ''I will be supporting the (funding) on the MX and the basing in Wyoming. I want to just be certain that the basing mode is advantageous.''