AGAIN THE EXCHANGE rate of the dollar has been sliding downward, conveying a message that Americans don't want to hear. The falling dollar is another signal that the country is borrowing too much and spending too much on current consumption. But with a presidential election campaign getting under way, nobody -- least of all anyone at the Reagan White House -- intends to do much about it.

The dollar sank low last May, then was buoyed by fears of trouble in the Persian Gulf and the possible effects of another oil crisis on Japan and Europe. That continued until the middle of this month when the latest batch of trade statistics swung the bankers' attention back to the American trade deficit. The dollar has not yet fallen quite to the levels of last spring, but it's down sharply over the past two weeks. Governments bravely talk as though they controlled the exchange rates but, in fact, they do not.

Consumers' spending, the government reports, rose rapidly in July for the second consecutive month. It went up much faster than the same consumers' incomes, which means that they were saving less than ever. The great consumption boom is continuing, and the United States as a society is not saving nearly enough to provide for the business investments necessary to keep the economy growing. The shortfall has been made up by foreign investors, but, some time ago, with the growing American debts and the comparatively low American interest rates, they began backing off. That's the underlying reason for the long decline in the dollar's exchange rate.

The Reagan administration's economic policy is going to be remembered for its easy toleration of its crashing inconsistencies. The most familiar example is the inconsistency of a balanced budget, higher defense spending and lower taxes. Similarly, the administration stands for high consumption, low savings, balanced trade and a stable exchange rate. Smiling affably, it refuses to choose among them. It prefers to leave the solution to the market. But the market, unfortunately, generally restores consistency through a recession. There is doubtless a better way to steady the dollar and balance American trade, but no one seems to be looking for it very hard.