AMERICAN political history is full of farm revolts. They seem to come almost every two years, which makes some sense when you realize that the prosperity of farms and the communities around them depend not on wages, which are relatively steady, but on profits, which are highly volatile. Yet the evidence is accumulating that the 1986 farm revolt is growing stronger -- ominously stronger for the Republicans. Farmland prices have plummeted and thousands of owners have lost their farms; country banks are tottering on the edge of insolvency.

The expectations that hundreds of thousands of people held and the hopes they cherish have been dashed. 1984 was a good year for Republicans and a record year for incumbents. 1986 in the farm belt may turn out to be a great year for the Democrats and a good year for challengers.

The problem for the Democrats is that the farm belt does not have as many seats as it used to, and this year there are not so many vulnerable Senate seats up there. Recent polls show Democrats have better-than-expected chances to win the governorships of Illinois, Iowa, Kansas and South Dakota. Democrats hope to raise serious challenges to such hitherto strong Republican congressmen as Vin Weber of Minnesota. But the key question of 1986 is whether the Democrats will win the Senate. The farm revolt helps their chances to gain seats in the two Dakotas, Oklahoma and, perhaps, Wisconsin, and to hold the seat Thomas Eagleton is giving up in Missouri. But few if any strategists on either side expect Democrats to win all of these, or to beat Bob Dole or Charles Grassley in the quintessential farm states of Kansas and Iowa.

And what if the Democrats were to gain control of the Senate by sweeping the farm belt? They could claim a mandate fr a different sort of farm bill, but would not likely have the occasion or the votes to pass it. On other issues the Senate Democrats would still face the problem the solution to which has evaded them as a minority: how to unite around plausible and attractive policies on major issues. The Republicans have good reason to feel queasy for 1986 and even 1988 about their sagging fortunes in the Mississippi Valley agricultural heartland of America. The Democrats have some right to feel elated, but they need to look out lest the gains they stand to make are limited and may not mean much elsewhere in the country.