BEDFORD, N.H. -- Presidential candidates don't have time to read, of course, so the message of Yale historian Paul Kennedy's new 677-page book, ''The Rise and Fall of the Great Powers,'' has to be reduced to bumper-sticker simplicity for their purposes.

But the aspirants for the White House in both parties would surely benefit from pondering the central lesson of Kennedy's survey of ''economic change and military conflict'' from the dawn of the 16th century to the end of the 20th. His point is best captured in these two sentences:

''The difficulties experienced by contemporary societies which are militarily top-heavy merely repeat those which, in their time, affected Philip II's Spain, Nicholas II's Russia and Hitler's Germany. A large military establishment may, like a great monument, look imposing to the impressionable observer; but if it is not resting upon a firm foundation . . . {of} a productive national economy, it runs the risk of a future collapse.''

Kennedy found a recurrent pattern in the five centuries he studied: through a combination of innovation and investment, a nation achieves a spurt of economic growth, such as the United States had in the first two decades after World War II. That economic power translates into expanded political leadership and responsibility.

As the growth nation expands its international commitments and obligations, it spends more and more on the bases, troops and weapons it feels it needs to protect its enlarged interests. The diversion of talent, energy and resources into the military begins to erode economic investment and innovation. And eventually other nations catch up and surpass the old power.

It may not be an iron law of history, Kennedy writes, but it is enough of a pattern that the challenge for American leaders today clearly ''becomes one of balancing the short-term security afforded by large defense forces against the longer-term security of rising production and income.''

''Balancing'' is the key word. That does not mean scrapping alliances or slashing the defense budget. It does mean taking a broader measure of national strength than the number of warheads, divisions or ships we can deploy, a definition that includes the productivity and growth of the economy.

As it happens, the conclusion that is implicit in Kennedy's book is intuitively grasped by most voters. I assert this on the basis of my own door-knocking in the past 16 months, but it is endorsed also by two separate public-opinion studies done late last year.

A national poll for the World Policy Institute found a 3-to-1 majority agreeing with the statement that ''Economic power is more important than military power in determining a country's influence.'' Similar majorities said they believe that while U.S. military power has grown in the past seven years, the economy has gotten ''weaker relative to other countries'' and ''America's industries are not geared to keeping up with the changes taking place in the world economy.''

By a similar margin, those polled cited these adverse economic factors as a greater threat to the future of the United States than the ''military strength of the Soviet Union.'' This survey, it should be noted, was taken just before the stock-market dive last Oct. 19 and before the U.S.-Soviet summit in Washington.

A similarly timed survey of southern swing voters (who had backed Ronald Reagan for President but a Democratic candidate for the Senate) found that a majority of that crucial electoral bloc believes that ''Ronald Reagan and the Republicans haven't done enough to make America strong by investing in education and by supporting American industry.'' According to the report issued by the Democratic Leadership Council, ''The Southern swing voters, while determined that America's defenses not be weakened and supportive of selective military operations abroad, do not . . . worry a great deal about America being strong enough militarily. . . . These swing voters hope that the next President will turn his attention to making 'American industries and workers more productive and competitive.' ''

The World Policy Institute study, which confirmed these findings about southern voters, found large majorities of conservatives and Republicans nationally share the basic view that economic power will be a more important determinant of America's long-term national influence than military power. As a campaign issue, therefore, this concern is not the property of one party or one candidate.

But as yet, no contender has put the proposition in clear terms. Those who advocate a continued military buildup tend to neglect the weakened economic foundation. Those who talk about investment at home say or imply that we can shirk our military burden. The message of history, as Kennedy reads it, is also a message these polls say the American people are ready to heed. For anyone who wants it in bumper-sticker terms, here it is: A strong America begins at home.