YOU CAN ASK people over and over whether they actually will take the trouble to vote, but the only poll that really counts is the one held on Election Day. Yet turnout can make all the difference. The 20-year trend is for turnout to decline, from 45 percent of eligible voters in the 1962 off-year election to 36 percent in 1978. For 1982, you can get predictions ranging from 42 percent to 33.

Some people always vote, and some people never vote; the crucial ones vote in some elections and not others. They include people who lean to both Republicans and Democrats.

In the prevailing negative political atmosphere, people seem more strongly motivated to vote against what they dislike than to vote for what they like. There are signs of a high turnout among black voters vehemently opposed to the Reagan administration and its works. The Republicans will also be hurt if the large number of evangelical-minded voters who flocked to the polls in 1980 react to the president's lack of emphasis and success on their issues, and stay home this year.

But will Democratic-leaning groups other than blacks vote in larger numbers than they did in 1978? Some observers see an increased turnout among white working-class voters. But Americans who see themselves as part of a working class are a smaller part of the total electorate than they once were. While there is evidence they are returning to their ancestral Democratic preference, it is by no means clear that they see the Democrats as having solutions for their problems. Are they hostile enough to the Republicans to turn out in larger than usual numbers?

On the answer to that question hinges much of the suspense in the 1982 elections. Democratic strategists hope for an upsurge of traditional working-class voters, particularly in the hard-hit industrial heartland. Republican strategists hope that such people will read the problems of their areas as resulting from more basic maladies than Reaganomics and that they will not see either party as offering a clearly preferable solution. The elections will tell us something not just about numerical support, but about the enthusiasm and esprit behind the different political forces in the country.