Why is it that West Germany, a country that has struggled rather sucessfully for 33 years to rid itself of the stain of Nazism, almost certainly will have as its next president a man who once was a member of the Nazi Party?

Why is it that country with hundreds of skilled postwar politicans-old enough to be wise, yet too young to have served that earlier cause-cannot find presidential candidates without what is discreetly called "a past?"

The answer, in part, is that West Germany's political parties have not looked very hard. Nor have they had to. But events in recent weeks here are forcing more Germans-and important voices in the nation's press-at least to ask the questions.

In effect, what is at stake in the selection of a new president next May is whether West Germany is leaving itself vulnerable for another five years to charges that it still does not fully understand the sensitivity to such things outside Germany or even among its own youth, and that it still has not entirely cleansed the wound.

West Germany has had former Nazis in high places before. The first postwar government of Chancellor Konarad Adenauer had one in the Cabinet and one in the chancellery staff. The chancellor between 1966 and 1969, Kurt-Georg Kiesinger, also had been a wartime member of the party.

But why now, so long after that period?

For the most part, people here did not know it was about to happen again until it was disclosed in the country's liberal press a few weeks ago that the leading candidate of the conservative opposition parties, Christian Democrat Karl Carstens, had applied for Nazi Party membership in 1937 and that it had been granted in 1940.

Then, in an effort to neutralize the issue, the conservative press revealed that the highly popular and respected current president, Free Democrat Walter Scheel-whom the Bonn government would like to see run again-also had been a party member beginning in 1942.

After the war, both men were given a clean bill of health by allied denazification panels and courts. They were described as having been only nominal members of the Nazi Party-which at one time had more than 10 million members-without playing an active role.

Nevertheless, there is more to it. The recent disclosures, or eminders, have struck a chord here. The situation is complex. It contains elements of normal politics in an abnormal situation, hindsight, and moral judgments that may or may not be fair to these two prominent politicians.

Some of West Germany's most influential publications, however, have begun to call for new thinking.

The mass circulation newsweekly Der Spiegel in a recent lead editorial said the Christian Democrats decided too soon in favor of Carstens. It is time to look for a younger man, they said, "without a past."

The magazine said there were many within the party and suggested the liberal Christian Democrat mayor of Stuttgart, Manfred Rommel, 49, son of the famous World War II field marshal.

Munich's respected Sueddeutsche Zeitung made a similar point and suggested Bavarian Culture Minister Hans Maier.

Even the conservative newspaper Die Welt, in a recent front-page editorial, called for rethinking the Carstens candidacy because Bonn's enemies, "open and hidden, would use every means of propraganda and agitation to attach the swastika once again" to Germany.

In effect, however, neither of the suggested alternatives, or anyone else, has a chance. Conservative party politics are now officially locked on to Carstens and he and the Bonn government risk a new wave of unwated international criticism.

Carstens is the key figure because he is almost certain to be president, even though Scheel, who recently indicated he is not interested in runnning again, is far more popular. The reason is that while the Social Democrate-Free Democrat coalition prevails at the federal level, the conservatives hold the edge in the special federal-state assembly that elects the president every five years.

Although real political power is held by the chancellor, the president becomes an important figure representing West Germany internationally.

By normal standards, Carstens would seem a likely candidate for many conservatives. He has been a major figure on the right wing of his party for many years and has been speaker of the lower house of the Bonn parliament since 1976.

Some of those pointing to his status as a former Nazi undoubtedly are actually afraid of his conservative stance on foreign and domestic civil rights issues.

Carstens' Nazi Party days, however, have created serious uneasiness among some of his critics, who view his decisions then as those of an opportunist. When he joined the party as a young lawyer, he said, it was because a judge had advised it would be a big disadvantage not to.

Carstens' candidacy also reflects generally weak leadership by the chairman of the Christian Democrats, Helmut Kohl, a charge that has dogged him throughout his time at the top. Sources close to Kohl say Carstens was not his first choice, that Kohl had doubts because of Carstens' close association with his party's most conservative wing.

Yet, these sources acknowledge, there was "no serious discussion of other candidates nor a serious look for one."

There were several reasons. Carstens wanted the job and was the major party figure seeking it. Carstens was favored by Franz-Josef Strauss, head of the even more conservative Christian Social Union in Bavaria, the small but powerful sister party of the Christian Democrats. Kohl consistenly has had difficulty in containing Strauss' wishes.

In addition, these sources say, once the left wing began attacking Carstens, there could be no thought of withdrawing him.

How the West German people will react, if at all, is uncertain.

Though Carstens and Scheel both indicated after the disclosures that they had never attempted to hide their past, there had been no public discussion of it here until now.

Before Scheel accepted the candidacy in 1974, the Sueddeutsche Zeitung reported last week, the question whether to mention his party membership publicly at the time was discussed only in a tight group, including then Social Democrat chancellor Willy Brandt and Free Democrat chairman Hans-Dietrich Genscher. They decided not to bring it up.

Chancellor Helmut Schmidt, asked recently about the recent disclosure especially involving Scheel, said he wa against starting the denazification process over again after 32 years. That is popular view here. He said Scheel nominal membership would prove no burden.

The denial of high office to politicians who have been cleared and have pursued careers since the end of the war also raises serious moral questions. Many West German liberals when cases of Nazi background came up, asked who among the critics was sure he would have reacted differently during the turmoil of Germany in the late 1930s.

Most people here, two-thirds of whom were born after the war or were children during the war, are tired of the Nazi label. Still, as a Stuttgart newspaper observed recently, "The risk of having chosen a man who might not be suitable for the presidency will have to be carried by the whole of the conservative union." And perhaps by the whole of West Germany as well.