Britain's new Social Democratic Party, already a force in public opinion polls and a growing presence in Parliament, was formally launched today with American-style hard sell as a modern, middle-of-the-road alternative to the political polarization accompanying the country's steep economic decline.

In press conferences here and in nine other cities, the new party's leaders -- nationally known former Cabinet ministers who defected from the major opposition Labor Party -- argued that sharp swings between union-based Labor on the left and Prime Minister Margaret Thatcher's ruling Conservative Party on the right have only exacerbated Britain's economic and social problems.

"Britain will not recover her national self-confidence unless we have a fresh start -- a different from of politics," said David Owen, former Labor foreign secretary and now leader of the 14 Social Democratic members of Parliament.

The slightly left-of-center Social Democrats declared that they want to accomplish "12 tasks," including revitalizing Britain's mixed economy with North Sea oil money, shoring up the welfare state, fighting inflation -- primarily through wage control agreements -- and eliminating class sex and race discrimination and regional divisions in the country.

Owen said the "essential element of our program" was changing Britain's traditional voting system of winner-take-all in each constituency, which favors two-party politics and one party majority rule in Parliament. Instead, the Social Democrats, along with their likely centrist ally, the Liberal Party, want proportional representation that would give minor parties more seats in Parliament and be more likely to produce consesus coalition governments such as those of many other European countries.

Roy Jenkins, a Social Democratic leader and former deputy Labor Party leader, told reporters that the new party strongly favors continued full British participation in the European Community and the NATO alliance, both opposed by left-wing insurgents who have taken over Labor's party machinery. The Social Democrats want to pursue nuclear disarmament vigorously, Jenkins added, but do not want Britain to disarm unilaterally as demanded by Labor's left.

Accelerating a trend in British politics successfully exploited by Thatcher in winning office two years ago, the new party is using media-oriented American techniques to get its message across to voters. About a quarter million dollars worth of newspaper ads and computerized direct mailings to tens of thousands of voters are soliciting members for the new party.

The Social Democrats already have struck a responsive chord with voters during what political analysts consider an unusually opportune time in British history to launch a new political party. In recent opinion polls, the Social Democrats have run neck-and-neck with the Conservatives and Labor.

More important, a centrist alliance of the Social Democrats and the Liberals has picked up support from more than 40 percent of the voters in opinion polls, enough to win or at leat become part of a coalition government if this momentum were maintained until the next national election in 1983 or 1984.

All but one of the 14 Social Democratic members of Parliament defected from Labor. They and 11 Liberals face 336 Conservatives, 255 Labor members and 16 others from small regional parties in the House of Commons, leaving Tatcher's Conservatives a 41 -seat majority