Every four years, in seeming defiance of nature's clock, the American voter gets younger.

By the 1988 presidential election, 61.5 percent of the voting-age population will be under 45 years old. It will be the youngest electorate in more than half a century, and the Baby Boomers -- the products of that fateful bulge in the birthrate that lasted from 1946 to 1964 -- are the reason. They are all old enough to be in the voting pool, and they've brought along their skepticism, anti-institutionalism, fondness for change and political volatility.

Some contours of the 1988 presidential campaign already reflect the tug of youth. Most of the candidates are young. None of the seven Democrats in the race is old enough to have fought in World War II; one wasn't born until the war ended. Their average age is 47.6 years. Their messages are young. On the stump, the-torch-has-passed generational appeals are in; the oratory of interest-group liberals -- a staple of Democratic politicians of Walter F. Mondale's era -- is out.

Baby-Boom politics, in short, is hot.

But nobody knows quite what it means -- and many experts suspect that behind the big numbers and pop theories there is less than meets the eye. The statistics show that while boomers have many distinguishing traits, voting has never been one. They vote as differently from each other as they do from their parents.

"The truth is that a lot of us who have written and talked about Baby-Boom politics have gotten away with murder," confesses Thomas Kiley, a leading Democratic pollster who has made a specialty of studying the famous age group.

Yet Kiley still counts himself among the minority of political theorists (themselves mostly boomers) who think a boomer voting bloc is ready to emerge. "I think there is a generational identity that will show itself in 1988 . . . and organize around a candidate," he predicts.

Some in this school argue that what will matter most in 1988 is the youth of the candidate, not of the voter. They see voters of all ages inclined to replace Ronald Reagan with a much younger man -- just as in 1960 they replaced Dwight D. Eisenhower, then the nation's oldest president ever, with 43-year-old John F. Kennedy. Predictably, this theory is a favorite of Democrats; their 1988 crop of presidential candidates is 11 years younger, on average, than the GOP field.

In a poll taken last month in the Washington area by Peter Hart Research for WJLA-TV, respondents said by 50 to 29 percent that they would prefer the next president to be in his 40s rather than his 60s. Among respondents age 30 to 39, this preference skipped up to 57 to 20 percent.

Other pollsters, however, say that question posed in the abstract will always yield a disposition toward youth. "You would have gotten pretty much the same numbers in 1979, and look who we elected," said GOP pollster Richard Wirthlin. "You can just as plausibly make the case that what people will want after Reagan is a competent, experienced hand," said Robert Teeter, another Republican pollster. "The boomers, in particular, are very performance-oriented."

Theorists of boomer politics argue that generational empowerment will be a winning message for 1988. "There is a feeling of coming into one's own," said Sen. John F. Kerry (D-Mass.), 43, a Vietnam war veteran and critic. Boomers "understand that generationally this is their moment to exercise some leadership . . . . There's a window of opportunity and it doesn't last long."

Sen. Joseph R. Biden Jr. (D-Del.), 44, is the leading apostle of boomer politics in this year's presidential field. "My generation of Americans is being summoned," he says repeatedly on the stump. But he's not the first to tap the vein in a presidential race; that distinction belongs to Gary Hart -- in 1984.

"We acted as a generation against racial hatred . . . to stop the war in Vietnam . . . to demand a halt to the pollution of our land," said the voice-over in one of Hart's 1984 TV commercials, as images of civil rights marches, Vietnam war dead and antinuclear rallies flashed on the screen. "We acted and we did overcome. But now our promise is slipping away. The politicians of yesterday are trading our future by asking our price instead of challenging our idealism."

Several problems with this approach suggest themselves. For one, boomers have been notoriously late for everything, from marriage to voting. They may not yet be ready to respond to a summons to lead. "You could very well run into a 'What you mean, We?' reaction out there," said one Democratic strategist.

Second, boomers were practically reared on television commercials, and they tend to be cynical about sales pitches aimed explicitly at them. "When they see all these TV spots using '60s music to sell them cars, they know exactly what's going on," said William Carrick, campaign manager for Rep. Richard A. Gephardt (D-Mo.).

Finally, there is the risk of seeming to set one generation against another. "These generational appeals put a lot of people off," said Ann Lewis, a strategist for Jesse L. Jackson.

Some candidates are crafting policy proposals to appeal to boomers. It's no accident that Democrats are giving education issues top billing. Boomers are finally starting to have children.

But it's a Republican, Pierre S. (Pete) du Pont IV, who has come up with the most explicit boomer plank so far -- a plan that would give young workers the chance to opt out of Social Security by investing instead in IRA-type retirement accounts that, according to many experts, would bring a higher return on their investments. Du Pont's plan would protect the Social Security benefits of all who choose to stay in the system, but critics, led notably by Rep. Jack Kemp (R-N.Y.), argue that undermining the universality of the system is the first step in destroying it.

That Kemp position is somewhat at odds with Kemp's free-market philosophy, but the New York congressman also knows the demographics of the votes he needs now. GOP primary voters are generally not boomers; most are in their 50s and 60s, said his press secretary, John Buckley. He argues that there is a settled political consensus on Social Security, and sees no need to upset it for a plan geared mainly to the young.

That raises another red flag about the political clout of boomers -- they don't vote as habitually as their parents did when their parents were their age. Although 61.5 percent of the voting population will be under age 45 in 1988, only about 50 percent of the likely general election turnout, and an even smaller slice of the primary electorate, will be. (Boomer turnout has been increasing over the past decade, however, as more boomers move into their 30s, when the habit of voting typically takes.)

Even if there is little evidence that boomers vote differently than other age groups, they are distinct from their parents in many important ways.

They are part of an extraordinary birthrate bulge -- the pig in a python, as it's known to demographers. Throughout the nation's history, birthrates generally have followed a steady decline. At times, usually after a war, they pop back up -- but only for a few years. The post-World War II birthrate boom broke all the rules. It lasted for an unprecedented 18 years, from 1946-64, and yielded a record 76 million babies. The boomers' parents had seen the nation emerge from a depression, win a great and just war, help rebuild Europe and Japan and enjoy a suburbanized age of plenty. They reached their child-bearing years at the apex of the American Century. Many of them grew up believing in heroes, government, institutions.

The boomers did not. Vietnam, Watergate, stagflation, hostage humiliations -- the list is familiar. Boomers tend to be antiheroic, apolitical and post-ideological; they don't affix themselves easily to labels. They are, however, rigorous consumers. They are the best-educated generation in American history, and they demand performance -- in their politicians no less than their Sonys.

Consider: In March 1986, President Reagan had a whopping 78 to 20 approval rating from the 18-30 age group. A year later -- with the Iran-contra affair having intervened -- he had dropped to 50-50 among these boomers, a 28-point plunge. In this same period, Reagan lost only 12 points in approval from all voters age 45 and older, according to Washington Post-ABC News surveys. Boomers, in short, want to know what you did for them yesterday.

As distinctive as their habits of mind may be, their habits of voting are not. "Of all the things that separate people into voting groups -- income, gender, geography, race, religion -- age has always been one of the least important," says election analyst Richard Scammon.

Indeed, traits that separate boomers from each other may be more politically significant than the age factor that separates boomers from other groups. For example, in 1984, male boomers voted 12 percent more favorably for Reagan than female boomers did, according to ABC exit polls. The much-remarked-upon "gender gap" was almost exclusively a boomer phenomenon.

Pollsters have found that being unmarried and/or in the work force (both circumstances are more characteristic of boomer women than their mothers) correlates to Democratic voting preference.

Taking a different tack, Ralph Whitehead, a University of Massachusetts professor who conducts focus-group sessions among boomers, says boomer males "have strong egos and weak identities. They grew up in families where their fathers were the breadwinners. Now they're adults, and their place in the world isn't nearly as clear-cut."

He hypothesized that boomer men -- who supported Reagan in 1984 by more than 2 to 1 -- are drawn to candidates who exude strength, who bolster the macho identity in a confusing culture where women are breadwinners, too. The problem for Democrats, he added, is that at the presidential level, the surest way to demonstrate these character traits is by advocating a hard-line foreign policy. But Democrats, for the past two decades, have been generally noninterventionists.

There are other variances within the boomer group that seem to have a political cut. Older boomers (those in their 30s) are 5 to 10 points less supportive of Reagan than younger ones (those in their 20s). The key here is when they were young. Old boomers grew up with the causes and excesses and enthusiasms and aesthetics of the 1960s; younger boomers came of age in the Me Decade of gas lines and get-mine, the 1970s. "The younger group missed the whole flower child experience," said Democratic pollster Harrison Hickman. "They're more materialistic."

It is a theory of Biden, among others, that there is a dormant '60s idealism waiting to be reignited. "The cynics believe my generation has forgotten," he says. "They believe the idealism and compassion and conviction to change the world are nothing but the long-faded wisps of adolescence . . . . But they have have misjudged us."

The fact that most boomers never joined in those '60s marches -- they either opposed them, were too young for them or ignored them -- does not necessarily discredit the argument.

In 1984, Gary Hart turned his primary campaign against Walter Mondale into a test of old versus new politics, and of idealism versus special interests, and did up to 20 points better than Mondale among boomers in several primary states. But in the general election, one-third of Hart's voters switched to Reagan over Mondale. They liked the incumbent's appeals to strength, prosperity and entrepreneurship.

In 1988 the Republicans will have to win the boomers all over again, most of their party pros acknowledge. Prior success is no guarantee of future success. They will have to convince suddenly skeptical younger voters that the emperor really does have clothes, and the course he set really was the right one.

The Democrats' challenge among boomers is more daunting, but may hold more promise for a bigger payoff. "If you want to capture the attention of and the commitment of the younger half of the electorate, you have got to make a case for a new public philosophy," said Whitehead.

Biden, among other Democrats, has been trying to articulate a philosophy that encompasses the idea of community, sacrifice, selflessness. In some ways, it is the antithesis of the individualism so prized by the boomers. But Biden is figuring that individualism has led to excess, that the boomers themselves know it best, and that they are ready for a change.

His problem -- the Democrats' problem -- is that when JFK made a similar "Ask not" appeal to the so-called silent generation that came out of the 1950s, he spoke to an electorate that believed enough in government to view it as the natural agent of community.

Democratic politicians of Biden's generation have shaped their careers around a recognition of government's limits. Now they, too, are calling their generations to the ramparts of community. But what are these ramparts built of if not government? They haven't said.