When Rep. Les Aspin (D-Wis.) took his turn at outlining his party's problems and opportunities to the newly formed Democratic Policy Commission recently, he had quite a bit more to say about the problems than the opportunities on his chosen issue -- defense and national security.

He pointed out that in the public opinion polls, the Republicans swamp the Democrats when respondents are asked which party they would entrust with the national defense. He added that most of the Democratic officeholders who have recently switched to the GOP have cited the defense issue as a major factor in their defection.

No other issue goes more to the heart of the Democrats' current troubles or is rooted as deeply in their recent history. A major factor in any recovery from their current malaise will be their ability to convert the issue from a political liability to an asset.

The Democrats' problem begins with the fact that they don't have a defense policy: they don't know what they want and are constantly perceived as responding to the Republican initiatives. In fact, there's no reason to think the Republicans are any more coherent or enlightened -- their approach is primarily to build more strategic missiles, the need for which is debatable -- but it has the virtue of simplicity, and in politics perception becomes reality. Aspin noted that the cumulative effect is that the Democrats seem to be opposed to everything President Reagan proposes while having no alternatives of their own.

As a matter of fact, opposing or at least closely questioning the major weapons systems of the past few years -- the B1 bomber, the MX, "Star Wars," chemical and nerve weapons, et al., is not a bad record.

If the Democratic Policy Committee can recommend a policy other than the current one of breast-beating about the need to somehow get "tough" on national security, one that can be debated and built upon, it will serve the party handsomely.

It can start by outlining the nation's foreign policy obligations and goals, the obvious first step in tailoring its military capability and striking the proper balance between nuclear and conventional forces. As former Sen. John C. Culver (D-Iowa) suggests, there is room for a policy of "steady, sustainable annual modernization of both conventional and strategic forces" plus a consistent effort on arms control.

A major reason for the Democrats' confusion, of course, is that they are leaderless nationally and have been for some time. Another is that they were divided over Vietnam as the Republicans never were -- the GOP constantly invokes George McGovern's presidential candidacy as a symbol of Democratic wimpiness on national security.

"Because they were responsible for the beginning phase of the war in Vietnam, they are apprehensive about defense spending and distrust the executive branch," says Alton Frye of the Council on Foreign Relations. "This was exacerbated by Watergate and the CIA fiascoes such as the attempted assassinations of Fidel Castro."

With Jimmy Carter came cancellation of the B1 bomber and Minuteman III missile and the stretching out of the Trident submarine system, Carter's shock at the revelation of the Soviets' behavior when they invaded Afghanistan and the national humiliation over the hostages in Iran.

Part of the Democrats' problem with the defense issue is the GOP's success in playing them like a violin on it for the past 40 years.

On the one hand, the Democrats have scrambled ever since World War II to head off GOP charges that they're weak on defense and soft on communism. The Americans for Democratic Action, the vessel of the party's liberalism was founded by Eleanor Roosevelt, Hubert Humphrey and others as an anticommunist liberal organization, but whatever they do, to the Republicans it isn't enough.

On the other hand, as recently as 1976 the Republican vice presidential candidate, Sen. Bob Dole of Kansas, indulged in his party's familiar castigation of the Democrats as the "war party" whose presidents have embroiled the United States in every war it's fought in the 20th century.

Along with the intellectual process of forming a policy is the political need to sell it politically, as Reagan has. No Democrat appears to be as sure on or to have internalized the issue as he has -- certainly not in the 1984 presidential election -- and therefore no Democrat is as able to communicate with the people on it.

"The Democrats are a party of two groups: the experts who have never run for office and argue over facts and programs, and the politicians who are of two minds on what they want," says Larry Smith, a former Democratic Capitol Hill defense expert now at the Kennedy School at Harvard.

"They go up against Reagan, who invokes parables, emotions and values and in effect they fight that fire with paper. What they need are politicians who can get the issue off the deputy assistant secretary level and run for sheriff understanding how a thematic, integrated defense posture can reflect their values, needs and fears and those of their constituents."