The US House of Representatives showed once again this week that when it comes to acting on weapons systems like the MX missile, caution is its guide. Only a few days before the vote to build 15 more of the 10-warhead missiles, House Speaker Thomas P. O'Neill Jr. had spoken confidently of wiping out all of the construction money for the controversial weapon.
The Massachusetts Democrat now concedes that the prediction was partly a strategic move to defeat the missile, but he also had reason to believe he was right. Seven months ago the House had come within nine votes of rejecting the missile, and nuclear-freeze forces had been working hard to corral votes to strike all of the $2.7 billion in construction money for 1985, leaving only research funds for the MX.
But when the electronic voting machine was turned on Wednesday night on the House floor, the majority took a middle road. By a margin of six, they shied away from the extreme stand of zero missiles and chose the middle road - 15 missiles, as proposed by moderate Democrat Les Aspin of Wisconsin and reluctantly endorsed by the White House.
It was hardly a ringing victory for the President, who was seeking 40 missiles for 1985, added to 21 already authorized. ''That's the way they do missiles up there'' in Congress, Mr. Reagan told reporters afterward. ''Someday maybe they'll get around to doing it right.''
For the House Speaker, the vote was a stinging defeat because it keeps the weapons in production and because the Republican Senate, friendlier to the MX, will still have a chance to increase the number.
''The only amendment that could have possibly prevailed was the one that did, '' the House Speaker concluded after the vote. Not only does the Aspin proposal cut the number of missiles, but it also would delay construction until next April to give the Soviets an opportunity to return to arms control talks.
The Aspin plan is ''a carrot-and-stick argument that doesn't guarantee one missile being constructed,'' said House Democratic whip Thomas S. Foley (D) of Washington State. Representative Foley says he likes the concept in theory, but he fears that the Senate will remove the carrot offered the Soviets when it acts on the missile authorization.
So volatile was the mood on the MX that switches on both sides baffled both the House leaders and lobbyists. Many who had backed the MX in the past voted against it this week, while some who had been considered firm opponents shocked the anti-MX forces by voting for the missile.
''We picked up a number of swing (votes), only to have people who were with us swing away,'' said Michael Mawby, lobbyist for the anti-MX group SANE.
O'Neill said his side lost 10 or 12 members that he had expected to have. So close was the vote that the Speaker is vowing to take another shot at the missile. The House could vote again before completing action on the Department of Defense authorization, which includes the MX money. The bill will come up for further votes during the middle of next week, giving the Democratic leadership more time to sway the waverers.
But the Speaker goes against formidable opposition. The White House mounted a heavy lobbying effort in the past week to win Democratic swing votes and hold tight to the normally well-disciplined GOP members. Only 17 Republicans deserted the party and voted to kill MX construction money.
There appeared to be many currents running through the House on the issue.
''I think a lot of people agonize over the MX as a kind of personal decision, '' said Foley, who has voted for the MX in the past but opposed it in the votes this week. Some members didn't like the idea of ''rewarding the Soviets'' by halting the missiles while the Soviets are still boycotting the arms talks, he explained.
Members face a variety of pressures from their districts. The nuclear weapons freeze movement may have played a part in the switch of California Rep. Ed Zschau (R) to the MX opposition. Meanwhile, Republican Nancy L. Johnson's Connecticut district has pro-freeze sentiment, and she has withstood White House pressure to oppose the MX in the past. But when the key vote to kill construction money came to the floor this week, she was absent.
Many of the votes for the MX came from Southern Democrats, and pro-MX forces turned around several North Carolina members who had voted against the missile last November. ''There's a certain political risk in voting against the MX'' in the South, an aide to a Southern Democrat said.
For some members it is partly a question of supporting local industries. Rep. Jerry M. Patterson (R) of California was once in the opposition column but switched. Defense contractors are major employers in his district.