Marking a turn toward a new era of younger political leadership, the Social Democratic Party's executive group today picked Hans-Jochen Vogel to run for chancellor in national elections promised for next March.

The choice of Vogel, 56, the party's opposition leader in West Berlin, represents an attempt to rally the party's disparate electorate of working-class voters, intellectuals and youth behind one of the country's most capable political figures. His competence and reasonableness is acknowledged by the traditional labor union core of Social Democrats. At the same time, Vogel has shown himself tolerant toward West Germany's active youth protest movement.

Although a celebrated ex-mayor of Munich who won high regard also as justice minister in the 1970s, Vogel is hardly a household name in West Germany. He conceded that his challenge to Bonn's new conservative chancellor, Helmut Kohl, would be an uphill fight.

"I am a realist," he told reporters. "It is not going to be an easy race. The starting position could have been better."

Vogel's unanimous selection followed a decision earlier this week by former chancellor Helmut Schmidt not to run again, for health and political reasons. Schmidt was ousted by Kohl in a parliamentary vote Oct. 1.

Citing Schmidt for "achievements recognized worldwide," the party's executive group today formally thanked the ex-chancellor, who in recent years was frequently at odds with major segments of his party on economic and security policies.

In a statement announcing Vogel's nomination, the Social Democratic leadership added an emphatic appeal to Kohl's center-right coalition to keep its promise of arranging new elections March 6.

"There are increasing signs," the statement said, "that the government cannot agree on an unobjectionably correct and politic proposal in line with the constitution" to hold elections.

Scheduling an election poses constitutional uncertainties that Kohl so far has not addressed publicly. He must choose betwen rigging a parliamentary vote of confidence to lose it deliberately, or proposing a constitutional amendment to give the Bundestag, or lower house, the right to call new elections. Both options risk the charge of political manipulation.

Vogel's only serious rival at first for the Social Democratic nomination -- which must be confirmed in January at a special party election congress -- was Johannes Rau, 51, the good-natured leader of North Rhine-Westphalia, West Germany's most populous state. Rau took himself out of the running after agreeing with senior party officials that his current post, as head of the party's last major regional bastion, was too important to abandon for a very difficult national race.

A southern Catholic in a predominantly northern Protestant party, Vogel first attracted national attention when, at 34, he was elected mayor of Munich. He quit that job 12 years later, in 1972, bitterly at odds with party left-wingers.

But he has since mellowed, curbing the intellectual arrogance of his younger days and turning into an advocate of more dialogue with agitated youth. As justice minister under Schmidt between 1974 and 1981, Vogel resisted demands for more repressive legislation that came with a wave of terrorist activities.

He was dispatched to West Berlin in January 1981 to take over as mayor in an attempt to rescue a party branch demoralized by corruption charges and ideological splits. His patient handling of squatter protests in the city and his interest in integrating radical approaches help explain why some party leftists, who once fought him, now regard Vogel as less rigid and more broad-minded than Schmidt.