An irony of President Reagan's MX victory in the House this week is that the pivotal figure on the administration's side was not a senior Republican but a liberal Democrat who made his first mark in Congress as a Pentagon critic.

Rep. Les Aspin (D-Wis.), 44, is a sharp-minded, glib, former Pentagon "whiz kid" with a master's degree from Oxford and a doctorate in economics from the Massachusetts Institute of Technology who was elected to Congress 13 years ago.

Last Tuesday, on the eve of the House vote, Reagan quoted Aspin at length in a signed article in The Washington Post in support of the MX intercontinental ballistics missile, a testimonial to the importance of Aspin's voice and to his transition from maverick to man of the center on arms questions.

"Aspin carried a lot of intellectual water for us," a White House aide said.

Aspin was sharply criticized by some other Democrats for the position he took on the MX, and agreed in an interview that the decision to support the president on the missile represents a unique and risky experiment for liberals like himself. But it would have been worse for the Democrats and for achieving arms control agreements with Moscow to oppose the president at this time, Aspin says he believes.

Aspin described the vote on the MX as part of a bargain. What the president got was the missile; what he gave was a far more explicit pledge than before to seek arms agreements with the Soviet Union. The MX, Aspin said, may well give Reagan the "leverage" to succeed in those talks.

He readily agreed that he and other Democrats who supported the MX could wind up with egg on their faces if the bargain with Reagan falls through. Once started, big weapons systems are hard to stop, Aspin said, and there is no guarantee that Congress will stop the MX in future funding votes if the president fails to fulfill his promises on arms control.

"What really is going on is a difficult experiment," Aspin said. "We've had experiments before, like the Social Security Commission, where you have a bargain between the legislature and the executive. But it's usually a one-shot deal, where they come together for one vote, one piece of legislation, Democrats and Republicans.

"But what you are talking about here," in the arms field where almost any action takes many years, "is something that has to continue over several congresses and several administrations. I don't know of anything that's been done like this before, where you are talking about a deal that you're trying to hold together over such a period of time."

As to the genuineness of the president's intentions, Aspin said "we are about to find out" in a first test as Reagan studies changes in the U.S. negotiating position at the strategic arms reduction talks (START) with the Soviet Union scheduled to resume in Geneva June 8.

Several such changes--along with deployment of 100 MX missiles now and work on a new and less threatening small missile for the future--were all recommended earlier this year by a bipartisan presidential advisory commission headed by retired Gen. Brent Scowcroft. Reagan named the commission when Congress rejected the MX last winter and has endorsed all its recommendations.

As for the political risk, it is there for sure, Aspin said, not so much in any immediate electoral terms but because many of his liberal colleagues who broke with him on this issue are ready to say "I told you so" if there is no arms control agreement as the 10-warhead MX missiles are deployed.

"If the administration goes ahead and does its thing, makes a good faith effort and nobody sandbags the small missile and the arms control process and the Russians simply don't buy it, well, we tried. We made an effort. But if this thing gets done in because somewhere along the line the Air Force makes sure the small missile doesn't work or somebody makes sure that our negotiating position is absurd, then," Aspin said, there could be serious political repercussions.

Aspin said he voted for MX for three reasons.

First, he said he believes it will help Reagan bargain with Moscow because the combination of highly accurate MXs and the Navy's new Trident II missile poses a serious long-term threat to Moscow's land-based missile force, and maybe the Soviets "will be willing to make some accommodation to that."

"The second reason is that I'm convinced it would be a bad position for the Democrats to be against the Scowcroft commission recommendations. If MX had lost because Democrats had opposed it, then comes November, 1984, and Ronald Reagan can say, 'Well, I certainly might have gotten an arms control agreement, but the Democrats in the House didn't give me the tools I needed . . . even after I appointed a bipartisan commission and lots of Democratic experts on defense said this was a good thing.' "

With those Scowcroft recommendations approved, Aspin said, "I think Democrats are now perfectly within their right to hold Reagan accountable and say, 'Okay, we gave you everything you said you needed, now where's your agreement?' The standard story is that the Democrats have two issues to run on against Reagan. One is economics, and that may be fading. The other is arms control, and I think you have to vote for the Scowcroft commission recommendations to keep that one alive."

The third reason, Aspin said, "is that this damn thing the MX isn't going to go away. Big weapons like this don't go away. Even if you had the votes to kill it now, a year or two from now the Russians will do something like invade Afghanistan and the right wing will ride back into town saying that the Soviets did it because they feel emboldened and politcally daring because they've got an advantage on us. And bang, we're back to building MX and God knows what else and at what cost, and we'd probably wind up with 200 or 300 rather than 100" missiles, he said.

"That's what happened with the B1 bomber. Carter killed it at $100 million apiece, and Ronald Reagan brought it back at $200 million apiece."

And what of the argument that putting the MX in existing missile silos vulnerable to Soviet attack will put a hair-trigger on nuclear war?

"I think it does," he said, "at least in the short run. Those missiles are vulnerable as hell. But all you can say is that you just hope a crisis doesn't reach those proportions," and that the overwhelming force on both sides will convince the superpowers it is better to bargain than to fight.

"We've got a potentially dangerous situation," Aspin said, but it would be dangerous without the MX. He said he believes the oft-cited White House contention that the Soviets would not have agreed to an anti-missile defense treaty in 1972 if it were not for the U.S. threat to build one.

"I felt no interest in voting for MX," Aspin said, adding that his colleague, Rep. Albert Gore Jr. (D-Tenn), "definitely did not want to vote for MX." Yet both lawmakers and some other liberals supported the missile. The key difference that separated these liberals from their colleagues such as Rep. Thomas J. Downey (D-N.Y.), who opposed the MX, "was the belief that you needed MX to leverage the Soviets" to get an arms control agreement.

As for his transition from House maverick to middle-of-the-roader, Aspin said it really was nothing dramatic. He is a fan of Carter administration Defense Secretary Harold Brown who, Aspin said, "had his head screwed on right, and I liked a lot of what he was doing." Brown supported the MX. In those Carter days, Aspin also bucked liberal attitudes and supported draft registration.