Joseph C. Harsch. Joseph C. Harsch is a longtime Monitor columnist.

PAT BUCHANAN'S startling victory over Bob Dole in New Hampshire was like a mid-summer stroke of lightning that suddenly illuminates the landscape. But the American political landscape it disclosed is not what the Republicans who won the midterm elections two years ago thought it was.

Republican victory in '94 was based on the assumption that the primary complaint of the great mass of Americans was of a federal government that costs too much and tries to do too much. The popular slogan was "get the feds off my back and out of my pocket."

The Republican presidential candidates unreasonably assumed that it could work for them too. Senator Dole, the party-establishment candidate, has been campaigning on the assumption that what the voters want in 1996 is still less government at lower costs.

But that is not what Mr. Buchanan has been talking about. He has ignored the standard Republican agenda of lower taxes and less welfare. He has not demanded, or promised, a balanced budget. He has ignored those issues which the Republicans in Congress have been assuming would get their party back into the White House.

Buchanan has been talking about something different. His campaign has assumed that the prime concern of the great mass of working-class and lower-middle-class voters is the decline in their earning power in general and worries about the loss of too many good jobs in particular.

The first inklings of the change in the political landscape came in those early polls in Alaska and Louisiana. Buchanan also ran better in Iowa than most expected. But when the candidates got to New Hampshire the warning was more than one little black cloud. It was a great big thunderclap. The idea that Buchanan could actually beat Dole in New Hampshire was unthinkable - until it happened. Every aspiring politician in either party had better understand why, because the next presidential election is going to be won or lost in that great working and middle class of voters.

The essential political fact in 1996 is that real income for large numbers of American workers has been shrinking steadily for some time.

The American economy is in danger today, but not from the old familiar complaints such as inefficiency. American industry today is more efficient, turning out more goods per worker/hour than do German or Japanese industry. This time the danger comes from shrinking buying power.

Buchanan's proposed remedies such as a return to tariff protectionism might well make matters worse. He appeals to the resentment and anger that is building up. But whatever else you may think about him, you must credit him with being correct about many people's real concerns.

The winners this November are likely to be the ones with the most plausible proposals to increase the real income of millions of Americans.