Candidates cringe at prospect of low turnout Tuesday. Both major parties make special effort to get their voters to polls

The speeches are over, the final burst of television ads has nearly run its course, and politicians across the country can only wait to see who shows up at the polls tomorrow. Turnout - how many people vote and what segments of the electorate they come from - will determine the outcome of a number of United States Senate and gubernatorial cliffhangers.

With Republicans straining to keep a grip on their Senate majority, Democrats hoping to hold on to their majority in the governor's mansions, and pundits alert for signs of a Democratic resurgence in the last two years of the Reagan era, the question of voter turnout has taken on unusual significance this year.

``Turnout is the sine qua non of this election,'' says retiring Sen. Thomas F. Eagleton (D) of Missouri. The battle to succeed him between between Democrat Harriett Woods and Republican Christopher Bond has been one of the scrappiest of this election season.

But voter participation at the ballot box Tuesday is expected to continue its near-steady decline of the past two decades. One study released last week predicted that three out of every five eligible voters - 105 million people - would fail to show up at the polls.

At both the national and local levels, Democrats and Republicans are waging campaigns not of just issues, ideas, and personalities, but of demographic studies, voter registration drives, and get-out-the-vote efforts.

``It's the most extensive I've seen,'' says Patrick Caddell, a pollster for the Democrats. ``It's a real test of whether this kind of technique can stand.''

The Republican National Committee has devoted $10 million to bringing Republican faithful across the country to the polling places on Nov. 4. Democrats are engaged in their own more modest get-out-the-vote drive.

In Idaho, in one of the most tightly contested Senate races in the country, state Democrats completed the most extensive voter identification project in that state's history in hopes of turning out large numbers of Democrats to help nudge Democratic Gov. John Evans to victory over Republican US Sen. Steve Symms.

The ultimate results of these efforts lie, of course, with the voters themselves. Virtually all predictions hold that voter turnout will be lower this year than in previous years. ``The only question is how much turnout will decline,'' says Curtis B. Gans, director of the Committee for the Study of the American Electorate, a Washington, D.C., polling organization that produced last week's study. ``If it falls below the 1978 level [37.5 percent of eligible voters casting ballots], it will be the lowest turnout in nearly 50 years.'' Part of the problem, many analysts say, lies in the negative campaign tactics in many of this year's local races. Some voters are reported to be repelled by the tactics of both sides in local races, and thus plan to sit out the election. ``If you throw enough mud in the pond, it makes it a pretty unattractive place to go swimming,'' says pollster Peter Hart.

Off-year elections such as this traditionally fail to arouse the excitement and, therefore, the turnout of presidential elections. A few areas with close elections may see healthy turnouts. In Idaho, state officials predict the highest turnout in 20 years, in part because of the presence of a hotly contested right-to-work referendum on the ballot. The general trend of decreasing attendance at the polls has been ascribed to a spectrum of possibilities: increasing electoral frustration with issues that seem overwhelming in their complexity, inevitable public complacency in a one of the world's most stable societies, even voter intimidation by complicated and awkward polling machines.

But this year's campaign, with its conspicuous lack of a national theme, seems to have met with a heaping dose of apathy. Bad economic times tend to bring voters to the polls - as in 1982 when people stung by the recession flocked to the polls in greater numbers than in the previous off-year election. This year, says voting analyst Richard Scammon, ``most people are feeling just fine.''