The muddle that passes for policy- making in Washington these days is understandable, if not laudable, provided you keep one factor clearly in mind. Most of the key decisions are being shaped by a calendar the people in authority are trying to keep hidden.

The circled date on that concealed calendar is Nov. 6, 1984, when the president of the United States, one- third of the Senate and the entire membership of the House of Representatives will be up for re-election.

Politicians never forget the next election date, of course, but until quite recently, Nov. 6, 1984, seemed a long way away. Coming out of last November's mid-term election, there was a near-universal sense that there might be a respite from short-term political calculations. Both the administration and Congress looked forward to a passage in which policy problems might be addressed in their own terms, not just for personal and partisan gain.

Everyone knew it would not last forever. In fact, there has been a broad consensus that by the time Congress returns from its 1983 summer holiday, right after Labor Day, the policy-making period will be over and politics will once again reign supreme.

By then, the president either will or will not be a declared candidate for reelection. If he is, his every act and statement will be examined for its political purpose and motive. If he is not, many of the key senators in his party will be scrambling to succeed him. As for the Democrats, they will be just a month away from the first endorsement decision in their nomination battle. The first caucuses and primaries will be barely 100 days away.

Recognition that the time for addressing long-term problems in a long-term way would be brief, the smart players have sought to move their issues onto the agenda now. Drew Lewis, the astute former secretary of transportation, got his highway legislation and gasoline tax increase through the lame-duck session of Congress before Christmas. The bipartisan deal-making that moved that bill to passage carried over to a jobs bill in the early weeks of this session.

The most striking example of successful exploitation of this historical moment was the passage of the bipartisan plan to guarantee the long- term survival of the Social Security system. Having exploited the issue shamelessly in the mid-term elections, the Democrats decided to look for a solution focused on the year 2000, not Nov. 6, 1984, and Reagan met them halfway.

The immigration reform measure, now wending its way through Congress, represents another timely effort to deal responsibly with a long- term problem, which, in other time periods, has been treated just as a political football.

But short-term considerations are increasingly intruding on the process. The panicky retreat of both the Senate and House from the bankers' lobbying and propaganda against interest withholding legislation is a classic example of pre-election jitters.

The broader questions of budget and taxes are also being whipsawed by short-term and long-term thinking. An outline of a bipartisan plan for curbing long-term deficits emerged in the wake of the Social Security deal, in discussions between House Budget Committee Chairman James R. Jones (D-Okla.) and Senate Budget Committee Chairman Pete Domenici (R-N.M.). But that plan was torpedoed by President Reagan. He made it very clear he would fight to keep his original tax-cut proposal in place for 1984, and let the deficit problem await a second term--or the next president.

Everyone--even Reagan--recognizes that eventually the deficits must be reduced or the economy will be crippled. But congressional Republicans are tempted to join the president in avoiding the pain of tax increases. They would like to believe the recovery now under way can be sustained through November 1984, despite the deficits. But they fear it will be cut short by a resurgence of high interest rates, leaving them vulnerable to defeat. Their hesitancy has stalled the whole budget process.

There are similar dilemmas for the Republicans on arms control and for the Democrats on Central America. On both issues, they are trying to guess what the world will look like in November of 1984. Can Reagan be reelected without an arms agreement? Do the Democrats face a trap if communists subvert another Central American government before that date?

One point should be clear. The closer we get to November 1984, the more strongly short-term political considerations will dictate answers-- or evasions--of long-term policy problems. The best chance we have to deal with those problems is from now to August. After that, most of them will not be faced seriously until 1985.