For a few giddy months just after it was founded in 1981, Britain's centrist Social Democratic Party topped public opinion polls. Pundits said it was destined to "break the mold" of national politics, then as now largely polarized between Laborites on the left and Conservatives on the right.

Instead, as it fights its first general election and with balloting only two weeks away, the Social Democrats are in danger of virtual annihilation. There isn't a single parliamentary seat that the party can be certain of winning. Even its "prime minister-designate" Roy Jenkins and two other widely known party figures--David Owen and Shirley Williams--are having to scramble to keep their seats.

Without them to lead, the remaining Social Democrats in Parliament would constitute an ineffectual rump.

The mystery is why in this society, which to outsiders seems inclined toward moderation in so many ways, the Social Democrats are not doing better. Made up mainly of defectors from Labor ranks, the party joined in an electioneering pact with the older Liberals, Britain's ever-small but well-organized party of the middle. Despite running candidates in all the country's 650 constituencies, the Alliance (as it formally is called) cannot seem to attract more than one-fifth of the voters in the polls.

The reason appears to lie in a mixture of personalities, policies and the predominance of the two main parties in setting the rules that run Britain's political system.

The Social Democrats, in particular, have been unable to shake an image of fuzziness symbolized by a much-satirized Jenkins lisp that turns "Roy" into "woy" and such stirring words as "breakthrough" into something fatuous-sounding, akin to "grapefruit."

The Social Democratic leader, chosen by party members last summer, is 62, a respected former chancellor of the Exchequer in a Labor government and then president of the European Community. He undeniably is a thoughtful statesman-politician, having what the British call "gravitas." In a television interview this week, he wondered plaintively, "Where have the great traditions of British rational argument fled to?"

"Unfortunately," wrote Chris Dunkley of The Financial Times after watching Jenkins' performance and noting its lack of fire or passion, "sweet reasonableness is just about the last quality that ever won an election." A lackluster Jenkins has dissipated one of the Social Democrats' greatest initial assets--the sense that it could bring something new and dynamic into conventional British politics that had grown stale.

Owen, who was foreign minister of the last Labor government, and Williams, a one-time Labor education minister, are far more effective as campaigners. Williams gave a flawless, off-the-cuff 40-minute speech to supporters at a school here last night, tailored to developments of the day and for this suburban community. But their personal stature has yet to overcome the party's otherwise bland reputation.

The Alliance's outstanding figure has turned out to be the Liberals' leader, David Steel. Boyishly youthful at 43, with an engaging, direct style, he is consistently rated the most popular of all party leaders in opinion polls.

Yet Steel's party had only 13 seats in the last Parliament and seems unable again to marshal his personal appeal into a national majority when voters decide for whom to vote. The Liberals have survived over the years by maintaining a consistent if relatively narrow base among genteel middle-class voters in select constituencies scattered around the country.

But the Social Democrats are struggling to forge something different, a classless coalition bound primarily by its disaffection with what is portrayed as the growing extremism of the two big parties. This is the sentiment that caused the 29 Social Democratic members in the outgoing House of Commons--28 from Labor and one from the Conservatives--to abandon old allegiances.

Their support in the electorate has proven to be soft, experts say, because it represents more a protest against existing government and opposition policies than backing for a clearly defined agenda to replace them.

The Alliance platform is, in fact, a detailed program for a mixed economy of state and private enterprise, including enough reflation to begin cutting unemployment. On defense, the parties call for a scaling down of Britain's nuclear commitments but stop well short of Labor's call for unilateral disarmament.

The SDP and Liberals found it relatively easy to mesh their party platforms since they basically occupy the same ideological ground.

As the campaign grinds on, the Alliance--and especially the Social Democrats--has started counting on a late surge of support if voters decide that Labor does not have a chance of defeating the front-runner Thatcher and turn to the only other alternative.

Some optimism remains. "We have a more complicated package of policies," said Peter Buron, the Social Democratic candidate from Watford, another bedroom community outside London. "They are not simplistic like the right and left of the other parties." As a result, they take more explaining, he said, and that is why people will come to the Alliance late in the campaign.

Williams, in an interview as she trundled from one stop to another, placed her hopes on the impact of television and radio. Under British rules, air time for the parties cannot be bought and is parceled out according to parliamentary strength. The proportion also extends informally to news programs. This meant, she said, that the Alliance parties got only a few minutes of exposure for every two hours offered to the larger parties.

However, once the national campaign began, those restrictions were revised, giving the Alliance 80 percent as much time as Labor and the Conservatives.

Probably the biggest single obstacle to Social Democratic fortunes, according to its supporters, is that in the British system, as in the American one, only winning counts. In many other parliamentary democracies, seats are allotted based on national vote totals rather than who wins in each constituency.

Thus, the SDP could theoretically finish a strong second in every parliamentary seat it is contesting and yet be wiped out. Not surprisingly, one of the firmest planks in the Alliance platform is a change to proportional representation. But to achieve that, the parties would have to take control of the old, first-past-the-post method. That, based on present reckoning, would require an election-day miracle.