Despite President Reagan's landslide reelection, shifts in Congress could make it harder for him in his second term to get what he wants on some foreign policy issues, particularly Central America, a variety of political and diplomatic analysts said yesterday.

Although foreign affairs took up half the presidential debates, they decided far from half the voters, according to polls before and after the balloting.

Americans voted on pocketbook questions in 1984, making only marginal shifts in a Congress that has been at loggerheads with President Reagan on many foreign policy issues, an apparent decision by voters that current policy needs no major changes.

"I see no mandate on foreign policy," said Senate Minority Leader Robert C. Byrd (D-W.Va.) "The White House will have real problems if it tries to push through a right-wing agenda."

Reagan has said he plans to make arms control his top foreign policy priority and in that he will have Democratic support, according to Byrd and others. But there will be new occupants for some influential chairmen's posts and the Republican majority in the Senate will be reduced from five seats to three.

In the House, the GOP pickup of at least 15 seats will not give Reagan the "ideological working majority" he wanted.

None of the new members were elected primarily on the basis of foreign policy views, although some who are leaving played key parts in the U.S. role abroad.

The MX missile was deployed only after Vice President Bush broke a tie vote in the Senate. Repeated votes to fund rebels fighting the leftist Sandinista regime in Nicaragua got narrower in the Senate as the year progressed.

The defeat of Sen. Charles H. Percy (R-Ill.) opens up the chairmanship of the Foreign Relations Committee, a crucial post governing foreign-aid spending targets, arms sales and ambassadorial and State Department political appointments. Next in line is newly reelected Sen. Jesse Helms (R-N.C.), who has promised his tobacco-farmer constituents that he will remain chairman of the powerful Agriculture Committee.

Conservative forces are pressing Helms to reconsider that promise and, if he does and if the rest of the committee approves him as chairman, the committee would take a sharp right turn and become far more activist.

State Department officials make no secret of their belief that Helms as chairman would be what one called "an embarrassing loose cannon" pursuing his New Right agenda in ways that could plunge the committee into chaos.

A determined foe of foreign aid, Helms opposes the Reagan administration program of expanded aid to Central America. He is adamantly against the move by Salvadoran President Jose Napoleon Duarte to negotiate with leftist guerrillas but firmly supports aid to Nicaraguan rebels. He is also considered the Senate's best friend of Taiwan.

If Helms decides to remain on Agriculture, Sen. Richard G. Lugar (R-Ind.) would be next in line to head the Foreign Relations Committee, but he has said he would rather be Senate majority leader. Helms has said privately that he would take the Foreign Relations post rather than see it go to the third man in the queue, moderate Sen. Charles McC. Mathias Jr. (R-Md.).

On the House side, the defeat of Rep. Clarence D. Long (D-Md.) will bring Rep. David R. Obey (D-Wis.) to the chairmanship of the Appropriations subcommittee on foreign operations, a post used by Long to wring human-rights concessions from the administration in return for passing its foreign-aid measures for Central America.

Obey, a certified liberal, is likely to take positions more critical of aid to Israel than did Long, although analysts in the Jewish community say they have a good working relationship with him.

The new Congress appears to retain the basically strong pro-Israeli inclination traditional on Capitol Hill. There could be a clash if the administration seeks Israeli concessions that many State Department experts believe necessary to resolve the Arab-Israeli conflict. So far, however, there is no sign that Reagan intends to move in that direction.

Similarly, any arms-control clash must await evidence that the administration is going to translate its rhetoric into specific programs.

Lastly, some foreign-policy issues not on the front burner may take on added urgency in the coming months and lead Congress to part company with administration policy.

That could happen regarding the Philippines where U.S. support for the government of President Ferdinand Marcos has faced increasing congressional skepticism or South Africa where the administration's policy of friendship for the white-minority government has engendered continuing controversy among congressional liberals.