''It is hard to think of any other politician today who can inspire the warmth and trust that she does.'' So said the Times of London in an editorial on Shirley Williams, whose resounding victory at the polls last week continues the steady progress of Britain's young Social Democratic Party. It struck us how seldom the combination of warmth and trust is singled out in the public arena these days, and how appealing it would be to voters. Whether in economically beleaguered Britain or anywhere else, a season of sacrifice calls to leadership seen as sensitively, justly, and reliably responsive to national and individual needs.
Was it qualities more than policies that brought Mrs. Williams a parliamentary seat in that by-election in the traditionally Conservative constituency of Crosby? She reportedly steered clear of being pinned down on policies during the campaign. She offered her party in its alliance with the long-established Liberals as a ''democratic alternative'' to the hard-line Tories presiding over high unemployment and the Labourites floundering under far-left burdens. Her success is being laid not only to her own charisma but to voters' seizing an opportunity to protest the status quo.
After all, she was only the latest to tap evident desires for change: the success rate of alliance candidates in local by-elections has gone up from 23 percent last spring to 67 percent after October's Croydon parliamentary by-election.
At Croydon an alliance candidate upset the entrenched Conservatives. Earlier Roy Jenkins, one of Mrs. Williams's cofounders of the Social Democrats, had lent impetus to the ''alternative'' by almost winning an upset in another by-election. At the moment Mr. Jenkins is seen as the most likely choice for prime minister if the alliance should maintain its momentum until a general election - though Mrs. Williams herself is now occasionally mentioned, too.
To maintain this momentum, however, the Social Democrats and their allies will have to depend on policies as well as protest. They cannot offer simply a negative alternative in hopes that the failings of the two major parties will keep on driving people away.
Mr. Jenkins has been floating some specifics, such as a tax plan to enforce industry compliance with wage and price guidelines. Mrs. Williams's new prominence ought to bring attention to her recent book, ''Politics Is for People ,'' which shows no lack of interest in policies even if she has not been trumpeting them on the hustings.
Her concern about left-wing extremism is combined with devotion to what might be called left-wing moderation. She sees a continuing major social and economic role for government, though she would decentralize it and promote participation all along the line. Whatever incomes policy there might be, she would increase local union bargaining and expand the issues from wages and hours to more emphasis on the quality of working life.
Long before the current report on measures to prevent future British riots, she was writing about the need for improvements in education and employment for the disadvantaged. She would begin the attack on unequal opportunities with children's earliest years. She argues, for example, that the most disadvantaged parents will not ordinarily bring their children to a nursery school, so the nursery schools have to actively seek such children.
No doubt much more about all this will be heard as time goes on. Shirley Williams does have something to offer, and to be debated, besides the warmth and trust that need no debate.