President Reagan's restrained reaction to the Soviet downing of a South Korean airliner has angered some of his most fervent conservative supporters. But the President's combination of hard rhetoric and reasoned action is likely to win political points for him where it may count a good deal more - among the centrists, or middle-of-the-road people, both in the United States and in Western Europe.
In the US, it may help Reagan shake off the image that some people hold of a man all too willing to seek confrontation with the Soviets, and dangerously so. And should he seek to run in next year's presidential election, that could add up to more votes for the President.
A public opinion survey sponsored by the Chicago Council on Foreign Relations and published earlier this year showed that while most Americans distrust the Soviet Union, they continue to support arms control agreements and other forms of cooperation with the Soviets. Majorities of those polled opposed grain embargoes and prohibitions against scientific exchanges with the Soviets.
In Western Europe, the President's restrained approach to the airliner incident may tend to demonstrate that Reagan is more reasonable and pragmatic than the trigger-happy cowboy that some Europeans consider him to be.
''Looking at it from a purely political point of view, I think the President gains a lot by reaffirming his position in the center,'' said Richard Scammon, a well-known analyst of public opinion polls who is director of the Elections Research Center in Washington.
''The reaction from the Congress seems to be that this was a measured response on the part of the President,'' said Mr. Scammon, who serves on the recently appointed presidential advisory commission on Central America. ''He's picking up support from Democrats.''
''The feeling is you have to make a measured response,'' he said. ''You don't shoot yourself in the foot with grain embargoes, which hurt us as much as they do the Soviets.''
Along with some other observers, Scammon predicts that outrage over the Soviets' shooting down of the South Korean airliner may well increase congressional support for defense spending and for Reagan's proposed MX missile program.
At the same time, some observers are convinced that the Soviet attack has lifted some of the pressure that was on Reagan from both Congress and West Europeans to make further concessions to the Soviets in the two sets of arms control negotiations now under way in Geneva.
One senior administration official predicted that in the end, however, the shooting down of the South Korean airliner will not make a major difference in US-Soviet relations. Officials have been arguing that the ''shootdown'' does not change Reagan's assessment of the nature of the Soviet Union but merely confirms it. Reagan has been taking the right approach all along and, therefore, does not need to alter it, they say.
This argument has upset some conservative who urge the President to adopt a harsher approach to the Soviets. Paul Weyrich, director of the Committee for Survival of a Free Congress, a New Right group, has called on Reagan to close US ports to Soviet ships, to expel Soviet diplomats, to cancel arms control talks with the Soviets, and to abrogate the recently signed long-term US-Soviet grain agreement.
The sanctions that Reagan decided on in the wake of the Aug. 31 Soviet attack on Korean Air Lines Flight 007 were relatively mild. He suspended negotiations aimed at renewing a US-Soviet scientific and cultural exchange agreement and at opening consulates in Kiev and New York. A tentative decision to renew an agreement on cooperation in transportation was canceled. The US is attempting to persuade other nations to suspend flights by the Soviet airline, Aeroflot.
Administration officials say that had the President decided to cancel arms control talks with the Soviets, it would have merely played into the hands of some of the more radical participants in the antinuclear movement in Western Europe. In West Germany tens of thousands are expected over the next few weeks to protest the planned deployment of medium-range American missiles.