One after another, senators took the floor during last week's MX debate to say that this would be their last vote for the missile.

It was a familiar refrain, heard in previous showdowns over the controversial weapon that defenders describe as essential to countering Soviet threats and detractors describe as an open invitation to escalation of the arms race.

But after 11 years, 30 votes and $13 billion of taxpayers' money, the missile survives as a 10-warhead monument to Congress' inability to say "no" to a president seeking a major new strategic weapons system.

Unless the House does the unexpected in its vote this week and blocks production of 21 more MX missiles, the focus will immediately shift to the fiscal 1986 request to double the MX arsenal by adding 48 missiles. Many skeptics say they think that the once-more-but-never-again arguments will be heard again.

"MX seems to lead a charmed life," said Sen. Lawton Chiles (D-Fla.). "It has more lives than the proverbial cat."

Sen. Dale Bumpers (D-Ark.) said, "I have been trying to think . . . when the last time a weapons system was defeated here. Weapons systems have gotten where they are just like Rasputin -- you cannot kill one."

Off the floor, a Senate aide was asked if a major weapons system had ever been killed by Congress. He thought a while and said, yes, there was the heavy-lift helicopter that Congress killed about 10 years ago. Pausing a second time, he added, "You know they're talking about it again . . . . The Army still wants it."

The three-day Senate debate that culminated in a 55-to-45 vote to release $1.5 billion for 21 MX missiles (in addition to 21 now in production) illustrated some enduring truths about Congress.

Presidents exercise enormous sway over Congress on sensitive issues, and lawmakers confronted by hard choices try to have it both ways.

Also, a weapons issue is rarely framed to compel a judgment solely on its merits. A skillful president can usually find some way to obscure the issue, as President Reagan did in arguing that a vote against the MX would reduce bargaining leverage at the just-opened arms talks with the Soviets.

In the case of this president and this Congress, the Senate vote -- even if the House votes the other way -- also underscored Reagan's extraordinary ability to persuade lawmakers to vote his way despite their strong objections to specific presidential policies.

Reagan had history running with him on the MX vote. No Congress, at least during the recent Armageddon-like concern over nuclear security, has denied a president a major strategic weapons system.

Congress may fuss over $900 claw hammers, $748 pliers and $640 airplane toilet seats, kill an ordinary weapons system and even lurch to the brink of definitive action on a major system, as it did with delaying votes on flight testing and the basing mode for the MX.

But rarely is there ever a clear-cut test vote on a weapons system once it has been begun. When there is one, other events, such as the arms talks in connection with the MX vote, can easily intrude, blurring the significance of the vote and making it easier for wavering opponents to find convenient cover to dodge the issue.

"There is a kind of reverence to strategic issues," said Sen. Gary Hart (D-Colo.), a leading MX foe. "Strategic issues are of a different order, a higher order . . . . It's survival; it's the whole ball game if things go wrong."

Moreover, Hart said, the "justification keeps shifting." First the MX missile grew in size in response to reports of Soviet advances, then moved through a series of controversies over its basing system that became intertwined with domestic politics and interservice rivalries, he said. Last year the missile was bait to bring the Soviets to the bargaining table; this year it is insurance to prevent the Soviets from getting something at the table free.

"A marvelous 10-year odyssey," Hart said.

After all this, the MX probably doesn't have the support of even one-third of the Senate, said Chiles, a former supporter of the missile who contends that his opposition is aimed at shelving the program until other options are explored.

"I have a feeling that there are not 30 people here who believe that we should have MX if we could ever get to a vote such as this on the merits. But I guess we never will," Chiles said. As for the arms control talks, "they could last five years, enough for 100 missiles," he said.

It wasn't just luck that brought the MX issue to the Senate floor a week after the arms talks opened in Geneva. Congress inadvertently created the fortuitous timing for Reagan by delaying release of the $1.5 billion until after March 1, when the president could petition Congress to vote again on the issue. In other words, Congress left the timing to Reagan, and Reagan exploited it by making his request and triggering the showdown votes just as the arms talks got under way.

Reagan's arms control policy, with its heavy emphasis on an arms buildup to force the Soviets to negotiate, gave him additional leverage in Congress. In almost Orwellian fashion, lawmakers were told they had to vote for more weapons to assure the elimination of weapons. For lawmakers who want arms control but fear appearing "soft" on defense and too conciliatory toward the Soviets, it was double jeopardy.

Many responded by opposing the MX with words, while supporting it with their votes.

Among these was Senate Minority Leader Robert C. Byrd (D-W.Va.), who broke with most of his Democratic colleagues to support the MX. He had wavered on the issue before the arms control argument became paramount.

Others such as Sen. Sam Nunn (D-Ga.), the ranking Democrat on the Armed Services Committee, threatened not to vote for the MX again, or at least not all the new missiles that Reagan wants next year. Some say they think that Nunn's support is critical, that he could carry enough votes to determine next year's MX quota.

But by then, unless the House blocks him this week, Reagan, who wants 100 missiles for deployment plus more than 100 spares, will have 42. At similar points in other weaponry controversies, Congress has shrugged and turned to new matters, as it did a few years ago when it moved from the B1 bomber to the MX. Already there are signs that the "Star Wars" antimissile defense system, formally known as the Strategic Defense Initiative, is beginning to capture Congress' attention.

Under these circumstances, Congress could go ahead with small but incremental increases in the MX arsenal or block further production, leaving what some lawmakers regard as a shrunken but potentially viable system.

Hart contends that the MX fight is worthwhile if it can result in fewer missiles being deployed. "In this case," the senator said, "it may be possible to be half-pregnant."