Austrian Chancellor Bruno Kreisky - the Socialist leader of a profoundly conservative country - faces the most severe test of his extraordinarily successful political career this Sunday when almost 5 million voters go to the polls in federal elections that will decide whether "Emperor Bruno," as he is sometimes called, will serve an unprecedented third term as their leader.

The 68-year-old Kreisky - who has already served longer than any other current elected West European leader - is by far the most popular political figure in neutral but pro-Western Austria.

His Socialist Party is also certain to capture more votes and seats in parliament than either the main conservative opposition, the People's Party, or the smaller Freedom Party which has a number of ultraconservatives in its ranks.

Kreisky, however, has repeatedly declared that he will serve another four-year term only if his party wins another absolute majority in parliament and that he would resign rather than lead a coalition with his opponents or a minority government that would force policy compromises on him.

Thus, the election here turns mainly on the chancellor's dominant personality and whether sufficient voters agree with the Socialist Party campaign posters plastered by the hundreds along Vienna's elegan Opera Ring, or boulevard, that say simply: "Kreisky: Austria Needs Him."

The answer to that question will also determine whether one of the most fascinating postwar political figures in Europe will be leaving the political scene.

The chancellor's political skill, statesmanship and fatherly charm have carried his reputation and influence far beyond what one would normally expect of a leader from a relatively small and politically neutral country.

Kreisky's counsel has been sought by many national leaders. He is a major figure in the Socialist Internationa, a close friend of former West German leader Willy Brandt and Sweden's former prime minister Olof Palme. He has brought Egyptian President Anwar Sadat and Israeli opposition Labor Party leader Shimon Peres together for meetings.

Although, he dropped his religion as a young man, he was born a jew in a country that was once among the first rank of anit-Semitic nations, something else that contributes to the mystique about him.

On the surface, there are virtually no issues in this election campaign. Austrians prize stability more than anything else and whether Kreisky or People's Party leader Josef Taus, 46, forms the next government, the general impression here is that nothing fundamental will change.

Yet to some, at least, the issue goes deeper. Austria, say the critics, needs young blood at the top both to tackle some needed reforms and to end the "illusion" that Kreisky unwittingly produces. Tha illusion, they say, is that Austria-because of Kreisky's international reputation-somehow is a biggre country than it really is.

"What Kreisky does," says Alfred Apyrleitner, the respected senior editor of Austria's Kurier newspaper, "is compensate for a national inferiority complex. In Vienna, we are always confronted with signs of the great past, of the Hapsburg Empire, yet we are so small now.

"In Kreisky, the people see a man with the broadest outlook, worldly attitudes and who even speaks a different language that most of them. You hear people say what a marvelous emperor he would have made, or president of the United States. But Austrians overestimate him and it may be better to forget this historical complex and neurosis. The empire is gone forever and our model ought to be that of a small country, someplace like Switzerland. We don't need national saviors."

Payrleitner was a strong supporters of Kreisky in the past and credits him with needed social reforms at the time he came to power in 1970 and in the following few years.

But now, he and some other critics argue, more reforms are needed in the nationalized steel industry and the economy and, he says, "The socialists do nothing. They just play the Kreisky superman card and nothing else. They have no vision, no ideology, no nothing. They only want to win."

The situations here is, in fact, much like the political scene in West Germany. Austria's big northern neighbor, except that Austria has virtually no political leftwing to bring pressure on politicians.

So Kreisky, although a Socialist, is much more conservative and popular than his party, as is Social Democratic Chancellor Helmut Schmidt in West Germany. Furthermore, Kreisky, like Schmidt, is blessed with relatively weak leadership in the major opposition party.

Taus, although an able and intelligent man, is a former banker and latecomer to politics who actually did better than Kreisky on economic issues on a recent television debate but has trouble conveying warmth or turning statistic into issues.

By a sort of gentlemen's agreement, the parties have refrained from bringing defense or foreign policy issues into the campaign to assure everyone that Austria's careful balancing act between East and West won't be disturbed no matter who wins. Taus, for example, believes Kriesky's strong stand against the policies of the current government in Israel amounts to a foreign policy without a mandate. Yet that is not debated here.

Taus worries that the Socialists have bought social peace at a price that has made the country's social welfare system staggeringly expensive and threatens to break down and put people out of work. He points to a federal budget deficit that has more than doubled in two years and foreign debt that has jumped from 10 to 26 percent of the gross national product in four years.

Yet Taus must make these points to a society which, on balance, is extremely prosperous and content. There are no strikes here. Unemployment is down to 2.1 percent, the lowest in Europe except for Switzerland. Inflation is down to 3.6 percent, half of what if was three years ago. The currency is strong, the economy is growing and the balancing of payments deficit is dropping.

Still, Kriesky is running a bit scared. He had a setback last fall when he asked his personal prestige on a nuclear referendum and lost. And, unless he gets a heavy voter turnout in Vienna, which still has a lot of old Socialists around, he could fall short of the absolute majority he seeks.

Perhaps to try and ensure a high vote in the Austrian capital, the chancellor, and Vienna Mayor Leopold Gratz, both made very tough May Day speecehs here last week in which they said what is at stake in this election is whether Austrian will turn back the clock to conservative rule and intellectual intolerance by putting conservatives and old ultranationalist parties back into power. The speech was meant to conjure up memories of the chaotic 1930s and, while it may be effective in scaring, old Socialists into voting, it was widely criticized here as a low and tasteless bit of campaigning.

Indeed, Kreisky could be forced into a somewhat compromising position if the vote is close, despite his pledge not to lead without an absolute majority.

In 1971, and again in 1975, the Socialists-behind a younger, somewhat more vigorous and reform-minded Kreisky-got just over 50 percent of the vote and 83 of the 183 seats in parliament. In 1975, the People's Party got 43 percent and 80 seats and the Freedom Party got 5.4 percent and 10 seats.

At a press conference here Friday, Kreisky in effect left open the door at least to discussions within the party and with Austria's president if he fell one or two seats short of an absolute majority.

At the moment, the race is too close to call, but many veteran political observers here feel the Socialists may just squeak through with 50 percent again.

Another potential alternative is that some of the younger politicians in the Socialist Party, especially the more flamboyant, 40-year-old, Harvard-educated Finance Minister Hannes Androsch, would step forward and try to form a coalition government. Although Androsh-a millionaire Socialist-is not terribly popular these days with Kreisky because of the growth, of his private accounting business while he was in office, he has the support of the powerful Socialist labor leader Anton Benya.

As for Kreisky, he had been having some trouble with an infection in his right eye, which also has hurt his appearance. Asked about it Friday, he brushed it aside in typical Kreisky style.

"Look at Moshe Dayan," he told a newsman. "He won a whole war with only one eye." CAPTION: Picture, BRUNO KREISKY . . . seeks absolute majority