THE DEMOCRATS, out of power in Washington, are in good spirits at their mid-term conference in Philadelphia. Twenty months after their unexpectedly severe defeat in 1980, they seem to have adjusted somehow to being the party out of power. Many of the people who originally invented the mid-term conference intended it to be a forum for the dissenters in a party in power. This year, it has become instead an occasion for a party out of power to display its unity. The Democrats, split for years over such issues as civil rights and Vietnam, are now divided only between rivals for a presidential nomination--the most gentlemanly rivals, at least so far, that we can remember.

The mid-term conference gives the country a chance to see how the Democrats expect to campaign. The theme this year is clear and not very surprising: opposition to the policies of Ronald Reagan. They seem confident that they will do well in November by attacking Reaganomics and supporting the nuclear freeze.

What the Democrats have not told us explicitly --parties in opposition seldom do--is how they would govern. They do not tell us exactly what they would do about taxes or Social Security or defense spending. Nevertheless, we think we can discern something important from the things that the Democrats are not saying. Almost entirely missing from the discussions in Philadelphia is what might be regarded as the unfinished business of the Carter era: national health insurance, a guaranteed annual income, progressive tax reform, labor law reform. In days of extremely high unemployment, there has been little mention of public jobs programs. Two years ago, at the Democratic National Convention in New York, the size of the jobs program was a focus of argument between the Carter and Kennedy camps. Now Democrats seem to assume that there will be no such program at all.

The Democrats seem to have given up on the idea of expanding government, and to be focusing instead on opposing Reaganite plans for cuts. A party that has long seen itself as an advocate of change now finds itself supporting what looks very much like the status quo in 1980. Like the Republicans during the New Deal years, the Democrats are now arguing that they could manage things more competently and would be more sensible and avoid the dangers of unduly precipitous change. That is not how Democrats are used to thinking of themselves, but it seems to be the role that they are assuming in politics today.