It says something unhappy about the political campaign that while the candidates slog back and forth across familiar terrain, the interesting ideas for American foreign policy of the 1990s and beyond arrive from other quarters. The most surprising such source is the Permanent Conservative Establishment, a collectivity that overlaps but does not coincide with the Reagan administration. Far from being drained or daunted by the exercise of power, this group turns out to have reserves of imagination and ambition. Its elite have learned something in the last seven years. At the least, they want another crack at power.

I refer specifically to the new Pentagon-sponsored scan of the policy horizon organized by two ranking defense intellectuals, strategist Albert Wohlstetter and Fred Ikle, who, as undersecretary of defense for policy, is the only currently serving official on a panel including Zbigniew Brzezinski, Henry Kissinger and other worthies.

The report is not everyone's cup of tea. Its readiness to contemplate certain levels of nuclear war-fighting cuts across a current running deep in our politics and our minds. Its picture of a world holding a potential for Soviet thrusts and ceaseless regional rumbles flouts the widespread craving for detente and easier times. Its emphasis on high tech, including precision conventional weapons to replace some nuclear weapons, has the specialists buzzing. Its call to concentrate scarce resources on likelier threats, not spread them out indiscriminately across many threats, defies a whole way of life at the Pentagon.

Theirs is the darker perspective of people who can conceive of an apocalypse, who are dead serious about defense. They do not hesitate to step forward as a commission on ''integrated long-term strategy'' and to launch a report under the forbidding title of ''Discriminate Deterrence.''

In fact, the Ikle-Wohlstetter gang is, without saying so, playing catch-up: trying to do well what the original Reaganites did poorly -- to coldly analyze the world and the ways it may change, to figure out the best uses for available resources, to accept a requirement to maintain public support and confidence over the long haul, to plan. The report tells Americans they must make hard choices of technologies, weapons and strategies. At the end of an administration whose implicit gospel is that there need be no limits, this is a breakthrough.

But will it do? As usual, other minds have been turning. Precisely at this moment comes along a historian cheeky enough to say his new book ''The Rise and Fall of the Great Powers: Economic Change and Military Conflict from 1500 to 2000'' is relevant to policy, and stylish enough to carry this presumption off.

Paul Kennedy's thesis is so simple one feels like apologizing for hailing it. He says that states which build power and empire can lose it all if they aren't prudent about paying for it. For this, you may ask, the professor writes a book of 677 pages? But Ronald Reagan the strategist seems never to have met Ronald Reagan the economic-policy maker. To finance its defense buildup and keep up American global commitments, his administration has gone into unbelievable debt and lost still more of the American economic-technological edge.

Ikle-Wohlstetter mean in effect to postpone a day of reckoning. Paul Kennedy warns, however, that the United States is risking a familiar ''imperial overstretch.'' Where Ikle-Wohlstetter seek to keep the American power position intact, Kennedy defines the task of policy as ''managing'' an inevitable erosion. With the authority of a historian surveying the ''age-old dilemmas of rise and fall,'' he summons the country to Walter Lippmann's call to balance commitments and power.

The Ikle-Wohlstetter panel, being history-minded, is no doubt prepared to grant the point. But its report is circumspect in pointing it out, not simply, I surmise, because the authors don't wish to pick a fight with the president but because American strategists are themselves not free from the failing of American ''exceptionalism'' -- faith in a free lunch. Candidate Reagan in 1980 preached eloquently about the importance of a strong economy to a sound strategy, but then he forgot it, creating the yawning gap which the Ikle-Wohlstetter report slips past and the Kennedy book addresses, as it should be addressed, head on.