Shirley Belmont is every incumbent politician's worst nightmare. A retired waitress from Huntsville, Ala., she says of Tuesday's election: "I'd love to kick the whole darn crew out."

She's also every incumbent's insurance policy: On Election Day, she doesn't plan to get near a voting booth. "It won't make no difference," she said. "Whoever you put in there is going to be a crook to begin with, so why bother?"

Belmont belongs to the only growing party in American politics -- the "party of nonvoters," as political scientist Walter Dean Burnham has dubbed it. She's one of an estimated 115 million adult Americans who won't bother to cast a ballot on Tuesday -- the most ever in a federal election, and roughly double the expected number of voters.

Voting has been in decline for 30 years, a slide that has persisted through war and peace, economic booms and busts, periods of contentment with politicians and spells of anger. While there is some speculation that the current sour mood may swell the ranks of protest voters on Tuesday, many political scientists think Belmont's reaction will be more typical.

"My guess is that voters will treat their disgust with politicians as a further excuse not to participate," said Benjamin Barber, a professor of political science at Rutgers University. "It becomes a rationalization for disempowerment. I suspect we're going to find out again on Tuesday that the American people's bark is worse than their bite."

"There are certainly enough new issues out there to grab public attention and propel turnout upward," said Curtis Gans, director of the Committee for the Study of the American Electorate. Gans cited the Persian Gulf, fears of a recession, rising oil prices, the savings and loan fiasco and the heightened stakes over abortion. But, he continued, "warring against" the prospect of higher turnouts are the decline in competitive races, the failure of the political parties to draw "clear lines of debate," and the heavy use of campaign advertising that traffics in "character assassination" and is designed to depress turnout.

According to surveys by Gans's group, voter turnout in the 1990 primaries was virtually identical to 1986's desultory level of just under 20 percent, and voter registration for the general election is about a percentage point below the 1986 level.

Turnout rates in midterm general elections always run well behind presidential years; the figures were 50.1 percent in 1988, the lowest turnout for a presidential election in 64 years, and 36.4 percent in 1986, the lowest turnout for a midterm election in 42 years. By contrast, turnout rates in 28 other democratic nations averaged 79.7 percent from 1969 to 1986, according to a 1987 Congressional Research Service report.

Who belongs to America's party of nonvoters? And how does their abstention affect partisan politics and government policies?

Voters and nonvoters are separated by three dominant demographic determinants: age, income level and education. A Census Bureau analysis of the turnout in the 1988 presidential campaign showed that: Someone over 45 years of age was nearly twice as likely to vote as someone 18 to 24.

Someone whose family income was above $50,000 was nearly twice as likely to vote as someone whose family earned less than $15,000.

Someone who graduated from college was twice as likely to vote as someone who did not complete high school.

These disparities have all grown more acute over the past three decades -- meaning that the United States, unique among Western industrialized nations, has a huge "missing" low-income electorate. In other countries, where political parties are both more vibrant and more class-conscious, low-income voters are mobilized to participate by parties whose platforms are aimed specifically at them. There is no equivalent here.

In the United States, according to a Washington Post survey taken last month, only 56 percent of nonvoters think there is any difference "I have never voted. I'm not real sure I'm thinking right. I'm not sure whether my vote would help the economy, or screw it up even more."

-- Quint Prestridge, Louisiana nonvoter

at all between the Republican and Democratic parties, and only 38 percent say they care which party's candidate is elected president.

Nonvoters are less inclined than voters to feel that the political system responds to them, and they feel less competent than voters to make sense of their political environment.

Belmont's words of cyncism -- "It won't make no difference" -- captures the first trait. Quint Prestridge, 26, an electrician's assistant from Alexandria, La., gives voice to the second: "I have never voted. I'm not real sure I'm thinking right. I'm not sure whether my vote would help the economy, or screw it up even more."

Given the low-income tilt to America's party of nonvoters, and the fact that low-income Americans have traditionally nested in the Democratic Party, the decline in participation would seem to have hurt Democrats.

But hard evidence of the partisan impact of nonvoting turns out to be thin and, at times, contradictory. One reason is that while poor Americans are disproportionately Democratic, young Americans are disproportionately Republican. Their low rate of participation hurts the GOP.

A National Election Survey taken by the University of Michigan just after the 1988 presidential campaign showed that, among nonvoters, George Bush would have defeated Michael S. Dukakis by 7 percentage points -- exactly his margin of victory among voters.

While skeptics point out that such post-election surveys suffer from a "halo effect" benefiting the winner, a series of polls taken over the past several years shows that the partisan identification of nonvoters is virtually identical to the partisan identification of voters. One explanation is that in the past three decades the political attitudes of lower-class citizens have been shaped as least as much by social issues, such as race, crime and traditional values, that favor Republicans as by economic issues that favor Democrats.

A deep recession, a patrician Republican president and a newly populist Democratic Party might change the equation -- but that's speculative.

What is not speculative is that while nonvoters and voters may have a similar partisan profile, they differ in their domestic policy preferences. The same NES survey that showed a 7-point gap between Bush and Dukakis among nonvoters also showed that nonvoters were more liberal than voters in eight areas of public policy: food stamps, social security, aid to the unemployed, child care, aid to the homeless, aid to education, aid to the elderly and space and science spending. Only in one area -- the environment -- did voters poll more liberal than nonvoters.

Surveys of this kind lead some liberal analysts to conclude that politics and policies would both be different if nonvoters voted. Frances Fox Piven and Richard Cloward argued in their 1988 book, "Why Americans Don't Vote," that "political attitudes would inevitably change over time if the allegiance of voters from the bottom became the object of partisan competition, for then politicians would be prodded to identify and articulate {their} grievances and aspirations."

That may be. But at the moment, 115 million nonvoters seem poised to do what they do best on a November Tuesday. They're staying home.