After leading the pack in virtually every opinion survey of Democratic voters for the past 10 years, presidential candidate Sen. Edward M. Kennedy (D-Mass.) is adjusting to the realization that he may lose next week in the first poll that counts.

Although he still says he thinks that superior organization and Democrats' disappointment with President Carter might give him a victory in Monday's Iowa precinct caucuses, Kennedy say that result would be a surprise. Near the end of his last trip through Iowa last week, he said it was a "reasonable expectation" that Carter would win there.

How does Kennedy, whom most "experts" considered a presumptive favorite over Carter just three months ago, feel about the rapid decline in his fortunes?

To the extent one can judge, the candidate these days seem philosophical about the possibility of an Iowa defeat.

"One of the things you learn in politics is that there are a lot of things that come up that you can't control", Kennedy said calmly in an interview last week. "What you do then is, you do the best you can and wait for things to change."

Kenneday today appears considerably less frustrated than he was a month ago about the impact of the Iran and Pakistan crises on his challenge to the incumbent. He said that "Iran and Afghanistan are still naturally the primary concerns in the minds of most Americans," but he said people are more willing to pay attention now to his speeches about domestic problems than they were in the first months after the hostages were captured in Tehran.

And while he expressed great disappointment about his failure to confront Carter face to face, he said that situation also will change.

"I can't believe that the American people will continue to permit 1980 to go by without a serious debate on foreign policy and domestic issues," he said. "And I think that will change things."

Kennedy pointed out that his presidential campaign proves that change is a constant in politics.

Last Nov. 7, when he announced his challenge to his party's president, Kennedy led Carter by a 2-to-1 ratio in national polls among Democrats. By December, he was far behind in the same surveys.

A CBS-New York Times survey released yesterday showed Carter leading Kennedy by a 44 to 34 a figure that buoys the Kennedy forces.

Last Friday, when the Des Moines Register came out with a poll of Iowa Democrats that seemed devastating to Kennedy's chances -- it showed Carter, Gov. Edmund G. (Jerry) Brown Jr. third at 4 points -- Kennedy did not look very devastated.

He joked with reporters and supporters about the poll figures. He has seemed much more relaxed on the campaign trail since that poll came out than in the months before.

This may be partly because the Register poll established him as a certfied underdog, a role he played with relish. As soon as the poll was released, Kennedy was before television cameras announcing that Carter "clearly much get 50 percent of above [In Iowa] or that would be a major setback." When the TV lights were turned off; he came over to a group of reporters and chortled, knowing that the poll had set an extremely difficult standard for Carter to meet.

In explaining his decent to underdog status, Kennedy does not indicate that he thinks this campaign performance has been a factor.

He attributed it instead to the "sense of national unity" in support of the president that developed after the Iran crisis, and to Carter's successful use of the tried and true "Rose Garden" campaign technique.

He says he is confident that those Carter assets eventually will diminish. And then, as a campaign staffer observed, "We'll be running again against a guy who managed to get himself a 19 percent approval rating."

With the first major test of the campaign five days off, why was Kennedy campaigning today in this picturesque New England town, 1500 miles from the Iowa action?

The answer, like many answers to political questions, revolves around organization and money.

Kennedy's workers in Iowa say they would feel obliged to turn out a big crowd if he were to campaign there this week. But they don't think people will turn out for political events twice in a week, and they'd rather have Kennedy supporters go to next Monday's caucuses than to just another stump speech.

In addition, Kennedy's Iowa budget will take him right to the limit permitted under Federal Election Commission rules, and the preparation involved in bringing the candidate back might break the bank.

And so Kennedy will have one more day in Iowa -- a series of airport news conferences around the state Friday.