London — As banners and rhetoric unroll a new season of politics, one underlying fact stands out:
British politics are becoming more American in style and approach.
''Specifically,'' comments veteran analyst John Cole, now political editor for the BBC, ''electoral success depends heavily on party leaders' popularity.''
National campaigning, especially on television, tends to swamp grass-roots, local activity at general election time.
Television will dominate even more as the fourth national channel opens in November and breakfast television arrives in January., Both cable and direct broadcasting from satellites are only three or four years away.
And the newest of the four main parties, the Social Democrats, uses US-style computer lists and mass mailings from a small central office in place of the more traditional system of a large party headquarters working through local branches.
The British electorate is still more homogenous than the American one. The country is smaller, more compact. But the trend toward a new style is unmistakable.
As all four parties plunge into their annual conferences, therefore, the political debate is largely about the standings of the various political leaders.
With the Liberal Party already meeting in Bournemouth, and the others to follow, here is the score card:
Margaret Thatcher: Riding high, with inflation falling and her Conservative Party still stressing, in almost presidential style, her leadership in the Falklands war.
Michael Foot: In deep trouble, his Labour Party split by an ever-deepening war between moderates and Trotskyites, and calls for his dismissal circulating in trade union halls behind the scenes. His rating in public opinion polls is remarkably low. His colleague Peter Shore is frequently mentioned as a possible successor - a younger, more attractive figure on television.
David Steel: A young leader who has studied the US scene. Leader of the Liberal Party, he knows that the alliance with the Social Democrats offers the best chance of achieving power since H. H. Asquith and David Lloyd George split the Liberals in the 1920s. But many pitfalls remain.
Some of his own rank and file oppose the alliance. He himself leads Social Democratic leader Roy Jenkins in the polls but faces heavy pressure from the Social Democrats to agree to Jenkins' leadership as the best way to win centrist votes in the country.
Mr. Steel, considered of ministerial caliber in Westminster, has never held office. Mr. Jenkins is a former chancellor of the Exchequer and one of the best-known political figures in the country.
Roy Jenkins: A figure of weight and presence, but yet to prove he is a leader of the future.
As a former colleague and now political opponent commented in private the other day, ''Roy has bottle (an English term meaning weight and status). He was the last chancellor to actually balance the budget.
''He is the alliance leader with the widest appeal. But he has not performed well in the Commons lately. Does he still have the drive? Does he have leadership to offer?''
At the moment, polls by Mori (Market and Opinion Research International) show the Conservative government well ahead. Labour has crept up into second place, and the alliance is, for the moment, third.
Prime Minister Thatcher can hardly relax, however. Unemployment is a massive 3.2 million and growing worse. Opposition politicians insist the real figure is 4 million, since many wives don't bother to register when they cannot find jobs, and half a million young people are on government-paid-for training programs that last for only one year.
The so-called ''Falklands factor'' was strong in June, after the campaign was won. But many observers say it is declining rapidly as voters return to more pressing concerns: standards of living (disposable income fell 2 percent last year), jobs, prices.
In the House of Commons, Labour's Mr. Foot has been struggling. The Tories stand back and watch the hard-left Militant Tendency Group nominate candidates for the next general election, and moderate Labourites demand their expulsion.
Mr. Foot's choice at his party conference Sept. 27 to Oct. 1 is unenviable. If he insists that hard leftists be expelled, he will gain credibility in the country but intensify the civil war within his party. If he allows only some lefists to be thrown out, he will buy time in the party but lose ground in the country.
As for the alliance, it must bring in proportional representation to ensure lasting influence, but both Tories and Labour are opposed.