West Germany's Free Democratic Party, the tiny but influential grouping that has determined the nature of nearly all of Bonn's ruling coalitions, chose a new leader today in an attempt to reverse its steady decline and stave off political extinction in the next national elections.

Foreign Minister Hans-Dietrich Genscher, who has headed the party for the past 11 years, relinquished his post as party chairman at an FDP congress in Saarbruecken today in favor of Martin Bangemann, the inexperienced economics minister who was the sole candidate for the job.

Genscher, who intends to continue serving as foreign minister, a post he has held for a decade, has said he is stepping aside as party chief because his government work will leave too little time to devote to the difficult chore of mobilizing enough support in the next two years to help the party survive the 1987 national elections.

Opinion surveys show that public backing for the party has slipped to 3 percent, well below the 5 percent threshold required to gain representation in the West German parliament. The party won nearly 7 percent of the vote in the 1983 national elections.

Much of the public's disaffection is attributed by party members to lingering dismay over the way Genscher deserted a 13-year coalition with the Social Democrats to form a center-right government with the conservative Christian Democrats in 1982.

Political commentators believe that Genscher must bear much of the blame for his party's erosion of support. His peripatetic travels -- he makes more than 50 foreign visits a year -- have left the party rudderless. Even Chancellor Helmut Kohl, whose close friendship with Genscher is a key foundation of the governing alliance, is said to have remarked that his foreign minister "would rather spend time in Burundi than with his own party."

Over the years, critics say, Genscher also has crippled potential rivals to the extent that a respected heir apparent never emerged among younger Free Democrats.

Ironically, Bangemann was forced out by Genscher as general secretary of the party in 1975 for suggesting an alliance with the Christian Democrats at a time when Genscher was still enamored of the coalition with the Social Democrats. Bangemann was dispatched to political exile in the European Parliament, but returned in good graces when Genscher decided to yank the party to the right in 1982.

Last year, as discontent with his leadership grew, Genscher staged a tactical retreat by announcing his retirement as party chairman months in advance and anointing Bangemann as his chosen successor.

After winning 352 votes to 31 at today's congress, Bangemann said the party must cease its ruinous internal feuding and concentrate on rebuilding its constituency.

Bangemann said that the Free Democrats would always tolerate dissent but that political wrangling must never again be allowed to damage the electoral fortunes of the small party, which faces three important regional elections in the coming weeks.

Left-leaning Free Democrats complain that the party has lost its soul and no longer offers a credible identity to potential voters, who appear confused by the party's eclectic mix of policies.

As a classic European "liberal" party, the Free Democrats traditionally have espoused free-market economics along with liberal social and human rights policies. This kind of profile, in the past, was designed to attract teachers, businessmen and affluent young professionals.

But the managerial class has turned toward the conservative views embraced by Kohl's Christian Democrats, while many young voters have flocked to the radical Greens party and their antinuclear, environmentalist crusades. The Greens have outpolled the Free Democrats in most recent state elections, displacing them as the country's third political party.

In his farewell speech today, Genscher urged the 400 delegates to spurn the policies of the welfare state. He said the Free Democrats should campaign for drastic tax cuts, curtailed trade union powers, and more support for small, independent businesses.