Washington — The sharp swings in public opinion that marked the Carter years have given way to a stability under President Reagan.
Experts describe the public's current mood more as one of balanced or fixed tensions, however, than of indifference or calm.
Nonetheless, these factors dominate the public's outlook at midsummer of the Republican's second year:
* Reagan rates no higher than Carter at the 18-month point. But the ''volatility'' in the public's view of Jimmy Carter - which plummeted 10 points, for example, in one month in early 1978 after the second Panama Canal treaty was ratified, then leaped upward 12 points after the Camp David peace agreements were concluded - has been followed by a relatively flat approval curve for Mr. Reagan. Carter, to the end of his term, remained a puzzle to the public. Reagan is a clearer and easier leader for the public to read, experts say.
* The fixed views about Reagan may protect him against popularity slides, but they also may make it harder for him to improve his standing. Opinion experts express surprise that Reagan got no boost from his highly publicized trip to Europe last month. Of six major opinion firms surveyed by the Monitor - Gallup, Roper, NBC, Yankelovich, CBS, Harris - two showed Reagan staying even in approval in polls taken before and after the trip, two showed him gaining a point, and two showed him losing a point. Previous presidents gained an average of five to eight points from foreign travel.
* The most striking change in public opinion that has occurred since the 1980 election is a sharp drop in public support for defense spending. Between 1978 and 1980, the steep climb in public support for arms outlays was the most singular feature. This has now been reversed.
The drop in relative support for defense spending to where it had been through most of the '70s can be taken as a plus for Mr. Reagan. The public feels that the arms buildup begun under Carter and speeded up by Reagan has restored the balance the public wanted between domestic and national security spending, explains Tom W. Smith, director of the National Opinion Research Center (NORC). However, the President may now have a harder time winning political support for a continuing hike in defense spending at the rate he wants. This explains why Pentagon officials are already scaling back their expectations for the 1984 defense budget.
To nations abroad, the return of defense spending to its familiar place among the American public's priorities should underscore what many opinion analysts have been saying all along, that the US has not embarked on a new bellicose path.
According to the NORC's latest survey just released, the US public's overall spending priorities have returned to the familiar pattern of the past decade. Social control or law-and-order items (crime and drug abuse) head the list for greater spending, followed by social welfare or social justice outlays (for health, education, environment, cities, minorities). National security spending (for arms, space, foreign aid) takes up the bottom of the list of items, along with economic welfare, such as payments to the unemployed - an item which rises somewhat in approval in recession and wanes in better times.
''Of over 200 additional items in our survey, no other item comes close in magnitude and direction to the drop in support for defense spending,'' says Smith of the NORC.
''Throughout the '70s and early '80s, the American public has generally favored domestic spending over national security spending. This order was somewhat upset in 1980 but by 1982 the traditional rankings had reappeared. To a large measure, the public has adjusted its attitudes to changes that have occurred and are continuing to occur in the distribution of federal spending. While support and opposition to defense spending approximately balanced each other out in both 1978 and 1982, the budget itself has changed notably. People are supporting the already greatly increased defense budget, but no longer believe that defense is being shortchanged.''
Reagan's rather fixed approval rating is linked to the overriding emphasis he and the American public put on the economy, some opinion experts say. A foreign trip will influence his position less because he has defined the measure of his success in terms of domestic economic results.
''The reason he's hanging on (in public approval) is chiefly because the people still aren't hanging the responsibility for the recession on him,'' says Greg Martire, vice-president of Yankelovich, Skelly & White. ''He continues to get points for his 'toughness' and gesturing, while people see him as consistent with their own outlook. Plus, intellectually, there isn't any opposing set of arguments raised to challenge him.''
''People are suspended,'' Martire says. ''They're willing to give him more time. The one number Reagan should most watch is the measure of blame people give him for the economy.''
''Reagan's made himself abundantly clear - to the delight of some people, to the dismay of others,'' says Andy Kohut, president of the Gallup Organization. ''The variation in views by demographic groups is so extreme - white vs. black, rich vs. poor, women vs. men - that it's stuck. The structure of opinion on Reagan is polarized. By contrast, Carter was always ill-defined.''
''General economic attitudes are not very changed from six months ago,'' Kohut says. ''Expectations for Reaganomics has slipped. Now, compared with January, fewer people have confidence in Reaganomics - but those who believed in it before believe in it even more strongly now.''
Opinion experts had expected Reagan's approval rating to rise after his European trip. ''We saw those numbers (poll results) come in after the trip and we were shocked,'' says Gallup's Kohut.
''Reagan's presidency is still a one-issue presidency - the economy. In the past, presidents have been able to turn to foreign issues when they're in trouble. But you can't take an economic issue out to lunch, the way you can a foreign leader.
''Still, should another extraordinary thing like the hostage taking occur, Reagan would get a big bang in public support.''
''Reagan got nothing from the trip,'' says Jeff Alderman, director of the ABC/Washington Post poll.