The special bipartisan commission that recently recommended deployment of MX nuclear missiles should "get back into business" and suggest changes in Reagan administration negotiating positions at the strategic arms reduction talks (START) with the Soviet Union, Rep. Les Aspin (D-Wis.) said yesterday.

In a letter to commission Chairman Brent Scowcroft, Aspin said the 11-member President's Commission on Strategic Forces "is just about the only body of people with the chance to bridge the ideological void" between the administration and many Democrats and moderate Republicans who doubt administration sincerity in seeking arms accords with the Soviets.

At a news conference, Aspin said that even though the administration may not like an outside commission recommending changes in White House negotiating positions it would be the best way to deal with the doubters, bolster declining congressional support for the MX and perhaps reach agreement with Moscow.

Aspin said that "If the administration wants to keep the bipartisan consensus on the deployment of MX . . . it has also got to have a bipartisan arms-control policy." He said the administration has made progress but still has "very little credibility" on arms-control issues.

Allowing Democrats, through the commission, to have a greater say in the negotiating stance, Aspin said, would give resultant bipartisan policy a better chance of surviving the 1984 election intact no matter who wins. This might end, he said, the constant discarding by one administration of its predecessor's policy in the complex arms-control field. President Carter discarded President Ford's initiatives, and President Reagan has overturned Carter's efforts, he noted.

Rather than writing Reagan, Aspin said, he wrote to Scowcroft to urge the commission to play a "prominent role" in arms control because, if the president reinvokes the commission, political suspicion would be raised.

Although Aspin said he has no assurance about how his request will be received, he made clear that it had been discussed with Scowcroft and the White House and that Scowcroft would hold a news conference within a few days to announce a new effort by the commission.

Aspin denied that his proposal would throw a monkey wrench into the Geneva talks even though recent progress there has been reported.

In June, Reagan ordered that the commission's life be extended but said he did not intend to have it alter internal White House positions at the arms talks.

Aspin said the United States would submit a new proposal at the next round of the START talks to begin Oct. 6. He said he hopes the commission can recommended changes in time for inclusion in that proposal and that the White House will accept the changes.

The START negotiations deal with reducing the number of intercontinental-range missiles and bombers on both sides.

The original Scowcroft commission of well-known former government officials and public figures from both parties was appointed by Reagan last year in an effort to solve the five-year question of what to do with the MX.

The commission recommended deploying 100 missiles, but also recommended a move away from the big, multiple-warhead MX missiles to smaller, single-warhead weapons, something favored by arms-control advocates. It also urged pressing ahead with negotiations but made few detailed recommendations in that regard.

Many congressional moderates have supported the MX on the basis of administration acceptance of the commission report and its pledge to press ahead on arms control. Aspin is a pivotal figure among them.

He noted that winning margins on MX votes in Congress recently have declined sharply and predicted that, if the moderates abandon the missile this fall, it will certainly lose in the House. He said it might even lose in the Republican-controlled Senate if the White House does not come through on promises made to key senators to include in a revised START proposal the so-called "build-down" plan for reductions.

Aspin said this situation gives the moderates "more leverage than ever . . . to nudge the administration toward a more bipartisan arms control policy" with Democrats making greater contributions.

He said the U.S. negotiating stance is still "murky" and has not been explained well to Congress or the public. He said, for example, that the administration timetable for big cuts in the Soviet arsenal of very large missiles is unclear. If the administration plans to force such cuts quickly, that will never lead to agreement, he said.

Aspin said he doubts that a START agreement can be reached before the 1984 presidential election, but that some agreement in principle might be reached similar to the accord at Vladivostok reached by President Ford and Soviet leader Leonid I. Brezhnev in 1975.