Inauguration Day is just a year away. Conceivably, the man being sworn in that day will be a true-blue conservative like Jack Kemp or Pete DuPont. If so, perhaps he will follow Ronald Reagan's example and bestow upon the Heritage Foundation the advantage of being "the president's favorite think tank."

But the polls suggest that isn't likely to happen, and as a result, Heritage is facing the prospect of losing its stature as Washington's leading cauldron of conservative thought -- the place to find out what the administration will consider doing next.

At the same time, its main competitors in Washington for conservative money, the American Enterprise Institute (AEI) and the Center for Strategic and International Studies (CSIS), are bolstering their staffs, sharpening their images and aiming to steal some of Heritage's thunder.

Welcome to the battle of the conservative think tanks. It's starting to look like a pretty fair fight.

For much of the past seven years, the battle has been lopsided in Heritage's favor -- at least in the bottom line sense of who gets the most money and attention. Heritage fellows, while hardly regarded as top-flight scholars, gained fame for their budget cutting proposals, "privatization" schemes and other contributions to the Reagan revolution.

Conservatives inside and outside of government flocked to conferences and parties at the handsome Heritage building on Massachusetts Avenue NE. The organization's income soared, from $5.3 million in 1980 to more than $14 million in 1986.

The other two think tanks weren't doing nearly as well. AEI, which had drifted from the right toward the political center nearly went broke as conservative contributions dried up, while CSIS -- which until last year was part of Georgetown University -- cut back its staff size.

Now the battle lines are shifting as the prospect looms of a moderate Republican or a Democrat coming to the White House.

New moderate-to-liberal think tanks, such as the Center for National Policy, have sprung up to join the better-known, heavily endowed Brookings Institution as the Democrats' favorites. And among the conservative think tanks, Heritage is consolidating, while its rivals are on the move.

At AEI, despite continuing financial woes, a young new president has given the organization a new sense of direction by hiring some big conservative names such as Richard Perle, a former assistant defense secretary. The new president is Christopher C. DeMuth, 41, who served as a soldier in the Reagan revolution at a top post in the Office of Management and Budget.

The leadership of CSIS, meanwhile, is raising funds more aggressively while seeking to position itself as the think tank that the next administration, regardless of party, will consult for its foreign-policy expertise.

The different problems and prospects facing the three groups suggest a stock market analogy to Heritage President Edwin J. Feulner Jr.

"CSIS is like a wholly owned subsidiary spun off from its parent corporation that has to establish its own new identity," he said. "AEI is a turnaround candidate. And Heritage is a strong growth stock that has to convince the market it's going to be around for the long term."

Such corporate terminology is by no means out of place in the business of "policy research institutes," as think tanks prefer to be called. The executives who run Heritage, AEI and CSIS sound just like other hard-headed businessmen when they talk about providing the sort of "product" (research, articles, seminars) that their "customers" (policy makers, corporations, the media) are demanding.

Heritage has been weighing its strategy for long-run survival for some time, Feulner said. "In early 1985, when Reagan was being sworn in, we said to ourselves, 'On Jan. 21, 1989, someone will be in the Oval Office and his name will not be Ronald Reagan,'" Feulner said. "So we'd better be prepared."

Feulner stopped expanding Heritage's permanent programs, instead offering fellowships for limited durations, so that it would be easier to control the institution's overhead costs. And he took advantage of the Reagan years by raising an endowment -- not nearly so rich as the bankrolls held by Brookings or the conservative Hoover Institution of Palo Alto, Calif., but sufficiently large to provide 9 percent of Heritage's annual income. Heritage will have no trouble prospering in the post-Reagan era, Feulner said, even if its growth slows. "Our target market is really Congress," he said.

Others said Heritage may be in for leaner times, especially in light of AEI's renaissance.

Under DeMuth, AEI is becoming a stronghold of neo-conservatism, and "a lot of funding for conservative think tanks comes from foundations that are really neo-conservative," said Edward H. Crane, president of the Cato Institute, which advocates libertarian policies. "So AEI is going to be a more aggressive competitor for Heritage than it has in the past." (Neo-conservatives mostly are erstwhile Democrats who believe that the party is insufficiently tough in opposing Communism.)

DeMuth fired almost the entire AEI foreign-policy staff and built a new department under former U.N. ambassador Jeane Kirkpatrick, one of the leaders of the neo-conservative movement. Besides Perle, the new foreign-policy group includes Frank Gaffney, who was Perle's deputy at the Pentagon.

CSIS, too, is threatening to reduce Heritage's market niche, albeit in a different way. Less ideological than its competitors, CSIS is planning to offer the next administration a "strategic agenda" on foreign policy and international economic affairs. "If we do our job right, this should be an exercise that is useful to any new administration, regardless of party," said John Vondracek, assistant to the center's president, David Abshire.

CSIS is hoping to distinguish itself by presenting forward-thinking analyses rather than specific proposals. Abshire, a former U.S. ambassador to NATO, argues that government is so compartmentalized that major issues tend to be handled piecemeal. "We will give policy makers a strategic vision," he said, promising that CSIS's research will have an "anticipatory, integrative, catalytic approach."

To "synergize" the work of the center's scholars, who include Henry Kissinger, Zbigniew Brzezinski and James Schlesinger, Abshire has hired Stanton H. Burnett, former counselor for the U.S. Information Agency, for the newly created post of director of studies. Meanwhile, CSIS officials said that with independence from Georgetown, they are able to knock on more doors for money because they no longer have to avoid tapping the university's most generous donors.

Still, despite the impending termination of the Reagan presidency and the intensified competition from AEI and CSIS, Heritage appears ready to weather the storm. Thanks to a sophisticated direct-mail operation, Heritage draws 37 percent of its income from individuals, a far higher proportion than its rivals.

Not only would those people be unlikely to stop giving to Heritage after Reagan is gone; they might even give more if a liberal Democrat wins the White House. "It's easier {to raise money} when you have a bogeyman," Feulner said. But he added that he doesn't relish the thought of Heritage thriving in the role of "President Paul Simon's least-favorite think tank."

"As someone once said, it's better to govern than to be in opposition," he said.

HERITAGE FOUNDATION: $14 million (1986) Industrials ..................37% Foundations ..................30% Corporations .................22% Investment income .............9% Publications sales and other ..2%

AEI: Approximately $9.3 million (1987) Corporations .................65% Foundations ..................21% Conferences, book sales, etc ..8% Individuals ...................6%

CSIS: $9 million (1986) Foundations ..................40% Corporations .................34% Individuals ..................11% Federal contracts .............6.5% Endowment income ..............5% Conferences, publications .....3.5%