LONDON, MARCH 3 -- After eight months of bitter and destructive argument, Britain's newest political party, the Social and Liberal Democrats, arose today out of the ashes of this country's political center.

The Democrats, as they wish to be known, are the result of a hard-fought merger between the venerable Liberal Party and the seven-year-old Social Democratic Party, who fought the last two general elections as a loose alliance.

Despite a new slogan -- "The Best for Britain" -- the party still has not named a new leader or defined some key policies, notably on defense.

More ominously, the party's premiere was as notable for who was absent as for who showed up. Holding his own news conference nearby was former Social Democratic leader and party founder David Owen, who vowed to continue to lead a rump, antimerger group under the Social Democratic banner.

With three of the old Social Democrats' five members of Parliament, including himself, and nearly half of its 60,000 premerger membership on his side, Owen said he would go on as the fourth of Britain's main-line political parties. His followers are "all determined to continue" the Social Democratic Party, Owen said.

The Social Democratic-Liberal alliance was supported by as much as 40 percent of the British population at the height of its popularity in 1985-86. But it began a steady slide early last year and ended up with only 22 seats in the 650-member Parliament after the June election. Within days after the ballot, Liberal leader David Steel announced he wanted the parties to merge into one, and Owen resigned, taking a large Social Democratic following with him.

Selection of a leader for the new party will not take place until this summer, at the earliest. Speculation is that neither Steel nor Robert Maclennan, a former leader of the Social Democrats, will be significant contenders. Much of the betting is on Liberal members of Parliament Paddy Ashdown or Alan Beith.