NATIONAL LEADERS including President Reagan and ranking Republicans and Democrats are locked in a high- stakes battle over how to interpret American public opinion.

The issue: whether the president's high approval ratings count more for political clout than the perception by members of Congress that his policies often lack support among voters.

Hanging in the balance is whether Reagan will be a powerful second- term president or a lame duck, ignored as Senators and House members work on their own agenda, at their own pace. Right now the president is losing.

What is so stunning is that the main actors in this drama are Republicans, and they are showing their disdain at a time when Reagan is almost universally regarded as being enormously popular. Popular presidents are supposed to be feared and deferred to, not confronted or ignored by members of their own party.

By acting so forcefully against Reagan, these Republicans are demonstrating that while the president may have great personal appeal, which no one disputes, they believe his policies are unpopular and that it is policy, not personality, that counts most with voters.

Starting this spring, the Republican controlled Senate has eliminated military spending increases that Reagan said were necessary, passed a resolution sharply critical of his plans to visit the German military cemetery at Bitburg, passed legislation calling for economic sanctions against South Africa despite Reagan's opposition and veto threats and is now leaning toward protectionist trade legislation in the face of Reagan's insistent criticism.

Twice Reagan has barnstormed for his version of tax reform; both times leading Republicans have gone out of their way to say that tax reform may be fine, but let's deal with other problems, such as the federal budget deficits and the trade imbalance, first.

In these fights, Reagan has done just about all the blinking. He agreed immediately to the military spending cuts, added a concentration camp visit to his itinerary in West Germany, was forced to draw up his own South Africa sanctions to head off Congress and has been backstepping on the trade issue.

Strong opposition to a president by members of his own party is rare but not exactly unique; Jimmy Carter was regularly savaged by the Democratic leadership. But the circumstances are extremely different today.

Carter won election narrowly and Congress rebuffed him before he could build a substantial national following. Reagan, on the other hand, is coming off the greatest electoral triumph in the nation's history, and is able to boast of very high approval ratings in the opinion polls.

The president has, in fact, trotted out the Gallup poll as a means of bringing the Republicans back in line, with White House press spokesman Larry Speakes noting that Reagan's approval rating, unlike that of most former presidents, has increased since his re-election. "He didn't get to 65 points with a song, a dance and nice smile," Speakes said.

So far the Republicans haven't budged. One of them, Rep. Richard Cheney (R-Wyo.), countered by discrediting the approval rating as a true measure of a president's popularity. "It would be a mistake to fall into the trap that, because he's so popular as a person, that this automatically translates into support for all his policies," Cheney said.

Even Reagan himself has made remarks suggesting that the rebellious Republicans are correct in their reading of public opinion, regardless of Speakes's comments. After a recent closed Cabinet session, Reagan was quoted as having admonished Republicans not to attack his administration for political gain of their own.

That is a stunning request. Republicans gain politically by attacking a Republican president with such a high approval rating? How? Only if the approval rating is not a trustworthy measure.

What we have here is the elevation of an old polling argument to the conduct of national affairs at the highest level. Some opinion analysts, myself included, have tended to discount the importance of the approval rating, holding that it is not a valid measure of the public's regard for a president.

Approval ratings are a very superficial, erratic tool. For one thing, they almost always shoot up when things go bad. John F. Kennedy's highest ratings came at the worst moment of his presidency, right after the Bay of Pigs invasion; his second highest came during the Cuban missile crisis. The public was not thrilled by either event; their support showed a rallying around the president, not an endorsement of his actions.

For another, when it comes to voting behavior, my own analysis over a period of years suggests that approval ratings are a minor factor. It is the "lesser of two evils" principle that guides the electorate the most these days. Walter Mondale would have won had he been seen as likely to do less damage as president than Reagan, and hang the approval ratings.

Mervin Field, the California pollster, has brought to my attention new evidence showing that the approval rating is indeed a flimsy measure. In polling during August, Field found 70 percent of the people he interviewed giving Gov. George Deukmejian a positive approval rating, an extremely high score. Yet in the same survey, Deukmejian placed behind Los Angeles mayor Tom Bradley, 50 to 44 percent, in a trial heat for a rerun of the gubernatorial election.

Deukmejian has been regarded as a highly popular governor, largely because he has consistently gotten high approval ratings in the polls. But private political polls as well as the one by Field show him trailing Bradley, and that must thrust into question both the governor's actual popularity and the use of the approval rating as a means of measuring it.

What is happening on the national scene is that the Republicans are making the same judgment about Reagan. Traditionally, there is no denying that a high approval rating alone has often been seen as a potent political weapon by the Washington establishment and political observers elsewhere. People equate a high rating with raw power. Not this time.

A recent Washington Post-ABC News poll, done at the end of July, showed the president with a 65 percent approval rating. But in it, the public gave Reagan negative ratings for his handling of the budget deficit, tended to side with Congress on the South Africa issue and had very mixed views on the direction of the economy and Reagan's policies toward Central America.

A poll in September showed his approval at 62 percent, only slightly lower, but with pessimism on the economy growing.

Virtually all polls for more than two years have shown opposition to Reagan on the key issues of increased military spending and cuts in social programs.

Twenty-two Republicans are up for election next year. Some no doubt are bucking Reagan because they think his positions are wrong. But they wouldn't buck him so frequently or so strongly unless they also thought it was good politics to do so.