IF YOU ARE wondering why there were so many negative ads and messages in the campaign that has just ended, ask yourself this: what positive message would you write for a candidate this year -- any candidate, any office? At first the exercise seems easy: wave the flag a little, talk about some past accomplishment, feature film footage of the candidate's family, talk about being close to the people.

But then the going gets tougher. If you're writing for a Republican congressional candidate, you can talk about the glories of a balanced budget and a strong defense. But you'll have to remember that you're talking to an electorate that is painfully aware that the Republicans haven't balanced the budget and have given up promising that they will, and an electorate that is concerned that the president and his men are too much given to dangerous saber-rattling. So you'll doodle for awhile on your legal pad, and then you'll come up with a good idea: a negative attack on your opponent.

If you're writing the script for a Democrat, the problem is similar. You can talk about the depth of your commitment to Social Security and the need to repudiate the evils of Reaganomics. But you'll have to remember that you're talking to an electorate that is painfully aware that your party's economics proved to be a disaster -- albeit a different one from the present -- and that you don't really have any new ones, and an electorate that suspects, with some reason, that something serious is going to happen to Social Security after the election, whatever the politicians say now. So you'll start embellishing the capital Fs on the word "fairness" which you have written over and over, and then you'll come up with a good idea: a negative attack on your opponent.

We have negative campaigns partly because these are difficult times and partly because voters express their views in such extravagantly negative terms that politicians, who want to show they share the ordinary voter's opinion, frame their own appeals in a similar tone. But there is another problem: the futures politicians feel they can realistically promise are not wholly positive. The public is too skeptical to let them get away with talking of benefits without costs, so they talk about costs--the costs of their opponents' policies -- and not benefits.

We may be stuck with such tactics in campaigns, but politicians, starting tomorrow, are freed from the discipline imposed by the proximity of an election. They are free to present to all of us, more candidly than they feel is comfortable in Octobers of even-numbered years, the policies they think the country should follow and the kind of future they can bring. We suspect the public is hungry for this kind of positive, candid vision, and that voters will reward such candidates more generously than politicians now suspect.