PORTSMOUTH, ENGLAND, AUG. 31 -- Britain's Social Democratic Party (SDP), formed six years ago with a pledge to change the face of this country's polarized politics, paved the way for its own demise today, voting at its annual conference to negotiate a merger with the Liberal Party.

An overwhelming vote favoring the merger talks followed two days of emotional SDP debate in this southern port city. It marked the culmination of long infighting after a poor showing by the SDP-Liberal coalition in the June 11 general elections in which Conservative Prime Minister Margaret Thatcher won a historic third consecutive term.

The promerger vote, coupled with the defeat of a move to allow an antimerger SDP minority to maintain the party name and take a share of its assets, was a major humiliation for former SDP leader David Owen. A former British foreign secretary and the party's most prominent member, Owen had led the antimerger campaign.

The vote means that the SDP and the Liberals will begin immediate negotiations over terms of the merger, which will be presented for a final vote at a conference next January. If approved, the two are likely to become a single, newly named party within the next year.

Until now, the two parties have called themselves the "Alliance," fielding joint slates of electoral candidates under a general policy statement while maintaining their separate identities.

David Steel, leader of the much larger Liberal Party, which has a centuries-old tradition but had fallen into electoral oblivion before the Alliance was formed, had called for merger after the last election.

In July, the SDP's 58,000 members voted on the merger, with 57 percent voting in favor of it.

In a television interview tonight, Owen vowed to continue his fight to keep the SDP minority as a separate party. "I think there are millions of people in Britain . . . who want to see it go on," he said.

Owen, one of five SDP members of Parliament, had argued that because of fundamental differences between the two parties, a merger would work to the electoral detriment of both.

Many of the speakers during today's merger debate expressed embarrassment and sadness over the fact that the party, formed as a middle-ground alternative for Britain's "reasonable" voters, had come to resemble the fractious Labor Party, from which the SDP split in 1981.

"We've become a circus," said one party delegate, while another called the SDP's public agony "this summer's entertainment."

Shortly after its formation, the SDP agreed to join an electoral coalition with the Liberals. When the Alliance won a series of by-elections and local races between 1983 and 1986, at the same time Thatcher's popularity seemed to be waning and Labor falling ever more into factional squabbling, hopes were high that the 1987 race would position the Alliance to challenge both.

When the votes were counted last June, however, the Alliance won only 22 percent -- down from 26 percent in 1983 -- and lost five seats in Parliament.