AN ENTREPRENEUR, seeing which way the market is heading, borrows and invests to cash in on the trend. Then suddenly things start going in the opposite direction. The cash stops flowing in, creditors get testy, old friends stop calling. It could be the story of a farmer, or it could be the story of Richard Viguerie, the conservative direct mail man. He is now being sued for some $4 million, has cut his payroll by more than half, has sold his Conservative Digest magazine and has his Falls Church headquarters up for sale. Some of his political opponents (not all of them liberals) are smirking. But there is no more cause for glee here than in the predicament of an overextended Iowa farmer.

For years Mr. Viguerie aggressively prospected for new donors, increasing his lists to some 5 million names. There was always some threat -- the Panama Canal giveaway, possible Carter Supreme Court appointments -- to point to. The sky is falling, the message would go, and nothing can stop it unless you send in your $15, today, not later this evening but right now. The sky-is-falling gambit is standard for all direct mailers.

But the pool Mr. Viguerie was tapping was not bottomless. Recent prospecting efforts have turned up few new contributors. Even some old lists have failed to produce expected returns. The weakness of direct mail is that, if people are reasonably satisfied, they toss the letter aside and spend the $15 elsewhere. Mr. Viguerie's own vocal dissatisfaction with the Reagan administration could not alter the fact that most Americans and most of his contributors agreed that, in the words of the 1984 Reagan TV ad, "it's morning again in America." Or, as a former Viguerie employee says, "There is no Democratic threat."

The big winners in direct mail fund-raising in 1985 were Common Cause and People for the American Way, who could point to the specters of corporate PACs and Jerry Falwell. The big losers were the Moral Majority, which has just changed its name to the Liberty Federation, and Mr. Viguerie. He is now seeking some nonpolitical work and negotiating compromises in lawsuits and with creditors. He has discovered that, in happy times, political direct mail is risky business.