Perhaps a droll Providence, casting around for some new chastening lesson for mankind, contrived the career of Margaret Thatcher to illustrate how little lasting safety there is in success. American Republicans should be taking notes.

The Labor Party, like the Democratic Party, is showing signs of having learned something. Labor may have learned that the public occasionally wants things from a socialist party, but rarely wants socialism, or even for socialism to be defined.

Shortly before joining the 1970s exodus from Labor, a former member of Parliament recalled advice he once received. When any argument about policy becomes too confused or contentious, just say, "We must have a socialist housing (or medical, or education, or whatever) policy." But do not be specific. A major figure in the great 1945 Labor government said socialism is whatever Labor governments do. But it is what Labor governments have said they were going to do that has caused alarm.

Today, however, Labor is expelling extremists, showing aggressive lack of interest in the traditional talk about nationalizing the "commanding heights" of the economy, and speaking of socialism as a vague ethical impulse. Labor's leader, Neil Kinnock, is a socialist who would rather be prime minister than be ideologically pure. He has the thing that saves socialists from socialism: ambition.

He does talk obligatory rot about unilateral disarmament, but that passion is necessary for his domestic tranquillity, his wife being eager to ban the bomb. However, even feminism has muddied socialism's ideological waters by challenging the traditional understanding of socialism as (in one analyst's words) "male, manual and muscular."

Labor's new temperateness is a tribute to two rivals, Thatcher and the Liberal Social Democrat Alliance.

Thatcher has (to use the jargon the left likes) given the proletariat false consciousness. She has tempted them onto the path to petit bourgeois consumerism: they want to own things -- houses, cars -- more than they want additional collective purchases of welfare-state benefits. They think more in terms of personal satisfactions than social equality.

Kinnock is respecting this reality. And the Alliance, to which some important Laborites defected, is poised to benefit from a recurrence of Labor's usual out-of-power reflex toward rhetorical radicalism.

Thatcher once said she aimed at "killing socialism." A measure of her success is the increasing plausibility of Labor as a party of government. There is an analogous danger for Republicans in Reagan's success in shifting America's political center, and with it the Democratic Party, rightward.

Considered as a party man, Reagan is the most important Republican since McKinley. He has changed, at least through two elections, the composition of the party by his ability to draw blue-collar and ethnic voters. Thatcher has changed the tone and, to some extent, the composition of her party through two elections. As one analyst puts it, she decided that, after generations of collectivism, the conservative mission is not to conserve what is best in British society but to change what is worst. The party's ethos is no longer that of the big businessman with a social conscience, but of the small businessman with social grievances such as inflation, unions, welfare-ism, taxation.

Kinnock is prospering by being militantly nice. The leaders of the Alliance have elevated affability and professional reasonableness to the status of political philosophy. They have decided the country is weary of Thatcher: sharp words and sharp edges.

It is said that f centuries Britain has sought unity in manners rather than ideas. Thatcher, who describes herself as a "conviction politician," takes ideas seriously, and in a divisive manner.

She is divisive in that she correctly thinks that some polarization is a consequence of leadership. In this she resembles Reagan. The resemblance ends abruptly with the matter of personal manner.

I know of no episode of Reagan's being rude to a colleague. There are many stories of Thatcher's wounding tartness. She is a workaholic and is weary, and fatigue does not give anyone a less jagged disposition.

Her forcefulness has increased what a former colleague calls "the grovel count" among her current colleagues, many of whom complain that she is brusque, peremptory and given to lecturing. But those traits have always been as conspicuous and as politically successful as Reagan's charm. Charm, however, wears better during a second term.

A former close associate says, "She is the only person I know who I have never heard say, 'I wonder whether . . . ' whether, in the natural rhythm of democracy, victories revitalize the vanquished, and whether character traits that produce highly personal victories also produce powerful reactions against them.