WASHINGTON (AP) _ President Reagan said the voters gave George Bush a mandate to continue the Reagan administration policies, but Bush appears likely to change course on some key issues, both critics and supporters say.

''I think, fiscally, circumstances will dictate that he has no choice but to be, if anything, a little more conservative than Reagan; in other areas he could be substantially more moderate,'' says Iowa's Republican Rep. Jim Leach, who supported Bush for the GOP presidential nomination.

''It is obvious that he will not permit Central America or Nicaragua to become the centerpiece of his foreign policy as Reagan did,'' says Lawrence Birns, director of the Council on Hemispheric Affairs, a organization often critical of the administration.

''You are not going to hear from him the same kind of fierce rhetoric about limited government that you have heard the last eight years,'' says M.E. Bradford, a professor of English at the University of Dallas and a writer on Southern conservatism. ''The sharp edge is going to be off a lot of things.''

There are other hints that winds of change may be blowing despite the much- heralded continuity of the vice president's succession to the presidency by election for the first time in 150 years.

As his national security adviser, Bush has chosen Brent Scowcroft, whose views have often differed from those of the Reagan administration. Scowcroft supported the single-warhead mobile Midgetman when the Pentagon wanted to scrap the missile and deploy the 10-warhead MX on rail cars instead.

And in testimony last year, Scowcroft called the administration's ''broad'' interpretation of the Anti-ballistic Missile Treaty, a translation designed to ease the way for tests of the Star Wars space defense system, ''unbefitting.''

Black civil rights activist Jesse Jackson met with Bush Nov. 30 and said he was able to discuss matters with him that he had never been able to raise with Reagan. He said they talked about anti-semitism and racism among other subjects.

The same day, Bush met with environmental leaders, one of whom, Jay D. Hair, president of the National Wildlife Federation, said the difference between the president and the president-elect was like ''night and day'' although Bush made no specific commitments. Major environmental groups opposed Bush during the campaign, but he repeatedly called himself an environmentalist, something Reagan seldom does.

In East-West relations, Reagan has consistently framed the basic conflict with the Soviets in moral terms. The president-elect speaks more often in terms of great power competition. It was Reagan, not Bush, who used to call the Soviet Union ''the evil empire.''

In the campaign, Bush, like Reagan, promised to pick judges who would interpret the law, not make it. He departed from anything Reagan ever said, however, in also promising there would be no ideological ''litmus test'' for judicial nominations.

With all of this, there are still more similarities than differences in their views, judging from their own statements and those of others.

''George Bush has gotten his mandate from the American people, the mandate to continue the policies that have brought peace and prosperity,'' Reagan said four days after the election.

Most insistently, Bush has restated the key pledge on which Reagan stood for eight years: No new taxes.

''For the foreseeable future, I think the people who are looking for him to change his mind about that are very foolish,'' said the conservative scholar Bradford.

Leach, however, said that ''circumstances and compromises could mean some movement'' on this issue although Bush will ''without doubt ... be much more disinclined to look at the tax structure'' than his Democratic critics.

Bradford and the Hemispheric Affairs Council's Birns agreed that in dealing with the Sandinista government of Nicaraguan President Daniel Ortega, Bush would seek a solution more acceptable to Democrats in Congress than the all- out military aid for the Contra rebels that Reagan has favored.

''I don't think the Contras will be abandoned,'' said Bradford. ''But I think he will try very hard to bring a lot of Democrats with him.''

''I think his instincts are the same as Reagan's,'' the Texas professor added. ''He would like to blow Ortega out of the tub.''

''We know that Bush is a cowboy on the subject of Nicaragua,'' said Birns. ''But it just doesn't make sense that he is going to strap himself to a policy that has failed.''

House Speaker Jim Wright, D-Texas, a foe of Contra aid, said after a post- election meeting with Bush that he was ''encouraged to believe that there is a search for a policy that will be acceptable'' both to the president-elect and Congress.

On the Middle East, many experts contend that the Palestinian uprising and other developments make the time ripe for more active American diplomacy. Sol Linowitz, who was sent to the region as a special envoy by President Carter, has suggested that Bush promptly name another such emissary.

Bush aides have said it is too early to make such a decision, since the new administration's Middle Eastern policy is still being formulated.

Reagan last week authorized a ''substantive dialogue'' between the United States and the Palestine Liberation Organization. Bush said he supported the decision.

''I think Bush will be committed to the future of Israel,'' Leach said. ''He also, I think, recognizes it is unwise not to listen to all sides. I think you'll see someone in the White House who understands more of the subtleties of the Middle East than we've had in recent years.''

Bush traveled to Israel, Egypt and Jordan in 1986 in a trip billed at the time as a mission to spur the peace process.

On other issues, Leach said, ''I look for progressive arms control, I look for a much more effective and principled use of international organizations like the United Nations. He set a little more moderate agenda in the environment, in education, in day care.''