London — Margaret Thatcher has set her sights on securing a prolonged period of Conservative Party rule unparalleled since the days of Churchill, Eden, and Macmillan.
After four years in power, the prime minister has called an early election for June 9 because she believes she has a better chance of winning it now than if she delayed the contest by even as little as a few weeks.
She had hoped to put off her decision until October, or even until early next year, when her full five-year term of office runs out. But faced with the prospect of rising inflation in the autumn, continuing high unemployment, and a move to unity among her leading political opponents, Mrs. Thatcher on Monday requested Queen Elizabeth II to dissolve Parliament at the earliest opportunity.
If the prime minister's strategy succeeds, she will gain another five-year mandate - bringing her party back toward its 13-year heyday in power from 1951 to 1964 under Winston Churchill, Anthony Eden, and Harold (''unflappable Mac'') Macmillan.
A critical element in Mrs. Thatcher's timing was the local elections last week. These showed the Labour Party's strength recovering - but not fast enough to promise the opposition under Michael Foot a victory if Mrs. Thatcher moved swiftly.
After weekend discussions at the prime minister's country residence, Chequers , an urgently convened Cabinet meeting, and a short limousine ride to Buckingham Palace, Mrs. Thatcher named June 9 as the day when she and her ruling Tories will confront the Labour Party and the Liberal-Social Democratic Alliance.
Most Westminster specialists concede that Mrs. Thatcher has chosen her moment wisely. Mr. Foot so far has failed to heal divisions in the Labour Party. At the local elections the Alliance performed below the level it was hoping for only a few months ago.
If the prime minister had waited, she might have had to confront new economic and industrial problems that would have given her opponents issues on which to campaign effectively. By choosing her moment now, she is trying for a second five-year term when neither Labour nor the Alliance is sure which issues will decide the contest.
Mrs. Thatcher appears to have few such doubts. Already Conservative Party printing presses are producing campaign literature underlining her ''resolute approach'' in domestic and foreign affairs.
Domestically, she has accepted rising unemployment as a price worth paying for achieving a promised liftoff in the British economy. Especially in the north of the United Kingdom, where unemployment is highest, Mrs. Thatcher's policies are unpopular - as the local elections showed.
But in the heavily populated southeast, which includes London, the Tories hope that their steady-as-it-goes approach will be accepted by most voters. Another fortuitous factor may be to the Tories' advantage: In many parliamentary seats recent changes in election boundaries are likely to favor the government.
Mrs. Thatcher's tough and successful line with strikers in many sectors of the economy is seen as another indication of her political strength. But Tory planners concede that the so-called ''Falklands factor'' - the reputation gained by Mrs. Thatcher during last year's successful battle with Argentina - is a diminishing asset.
These planners claim, however, that other aspects of Thatcher foreign policy will work to her advantage. While the Labour Party is pressing for a withdrawal from the European Community and abandonment of an independent British nuclear deterrent, Mrs. Thatcher is urging the reverse on both scores.
And 12 days before election day she will be at the seven-nation economic summit meeting at Williamsburg, Va., pressing her solutions to the world's problems. This is expected to give her valuable exposure at a critical time in the campaign.
A week after Williamsburg she is scheduled to attend a European summit meeting at Stuttgart, West Germany, where she will argue that Britain deserves a better budget deal from its partners in the Community than is currently on offer.
Throughout the campaign Mrs. Thatcher is expected to insist that Britain must play a central part in the NATO plan to deploy medium-range nuclear missiles in Europe. Labour Party opponents will reject that policy, and the Liberals and Social Democrats will try to water it down.
David Steel and Roy Jenkins, leaders respectively of the Liberal Party and the Social Democratic Party, said they welcomed Mrs. Thatcher's general election decision. But neither party did particularly well in the local elections.
Even optimistic forecasts suggest the Alliance is unlikely to gain more than 15 percent of the nationwide vote on June 9. Its best hope is that it will end up holding the balance of power between the main parties. But even that appears to be a remote possibility.
For Mr. Foot and the Labour Party the outlook is bleak. On the day Mrs. Thatcher called the general election, an opinion poll published by the British Broadcasting Corporation said that if Mr. Foot were not the Labor Party leader, his party would do much better in an election.
The deputy leader, Denis Healey, is more popular than Mr. Foot. But it is virtually impossible for the Labour Party to switch leaders at this late stage. In terms of personality, this will give Mrs. Thatcher an important edge in the coming campaign.
By deciding to go now, the prime minister has precipitated a short, sharp contest in which Labour will probably face an uphill struggle to assert itself in the public's mind.
Mr. Foot continues to insist that unemployment is the main issue. Mrs. Thatcher claims that she has brought inflation down, tamed the trade union movement, and prepared an era of economic stability in readiness for renewed national growth.