ELECTION POLITICS suddenly is no longer following the rules that had seemed for decades to be inherent in the Western industrial democracies. It was an iron law--or so we thought--that what counted was prosperity and above all employment. In both Britain and the United States, things have evidently changed. All the experts earnestly warn against drawing parallels between the two countries' politics, but the temptation is irresistible at least in this respect. In both countries, governments of the right came to power with explicit economic programs that have gone badly askew and sent unemployment to its highest levels since the Depression. But in both countries the opposition remains scattered and confused. In the United States, Mr. Reagan and his Republicans still hold the political high ground. In Britain, Mrs. Thatcher and her Conservatives have just won a tremendous victory.

In both countries, the explanation seems to be that after a long period of mixed signals the voters have been offered a clear and recognizable concept of a national majority. These majorities are very middle-middle class in tone--Mrs. Thatcher is shedding the Tory aristocrats with conspicuous speed--and not very responsive to racial and ethnic anxieties. They appeal to a kind of patriotism that carries echoes of an older nationalism.

One real difference between the two countries' politics is the doctrinaire radicalism that has become entrenched in some of the British labor unions. Its influence in the Labor Party steadily increased in the 1970s, eventually evoking a powerful and sustained national reaction to which Mrs. Thatcher owes much.

But while the scale of Mrs. Thatcher's victory is notable, it's important to observe that her popular vote on Thursday was slightly lower than in her first election four years ago. This time it translated into an astounding majority of 144 seats in Parliament because the opposition was splintered. Whether Thatcherite Conservatism now has a tenure of only one more term or something much longer will depend heavily on the ability of an opposition to pull itself together. The outlook for the Labor Party, now sunk deep in unhinged left-wingery, is not promising. There is more hope in the Social Democratic-Liberal alliance; although it won few seats, it came very close to equaling Labor in the popular vote.

Mrs. Thatcher has been rewarded for a firm and coherent view of her country's purposes that a lot of British voters recognized and liked. There is a lesson in that for politicians not only in Britain. She has also tested the proposition that even in a country with a deep historic fear of unemployment, people can be persuaded to accept it when the alternative is high inflation. Mr. Reagan is likely to see a parallel to American politics there. But perhaps an even greater and more triumphant vote awaits the politician who eventually finds a way to keep inflation down while employment goes up.