London — You'd think that Britain's 3 million jobless would all vote against Prime Minister Margaret Thatcher. Many will. But by no means all.
Time after time the unemployed say they will vote Conservative, even if they have been out of work for a long time.
A recent national opinion poll for the British Broadcasting Corporation covering more than 1,000 first-time voters (age 18) said a majority would vote Tory, even though 1.3 million people under 25 are registered unemployed today.
These people don't pin all the blame on Mrs. Thatcher. ''Most Western countries have the same problem,'' said a neatly dressed young man in a workingman's club in the Midlands.
Keith Smith, 22 years old and unemployed since March, has just been turned by seven companies in Walsall. He voted Labour in 1979 but will vote Tory June 9.
''Maggie has brought down inflation,'' he said. ''There have been too many governments swapping and changing. . .I think Michael Foot is scruffy. . .I don't want to see any defense cuts.''
To him, Mrs. Thatcher offers the sharp image of a leader. He simply dismissed claims of Labour and the Social Democratic-Liberal alliance that returing to more government spending is the answer.
Labour's recent fall in the public opinion polls results in part from Mr. Foot's image, but mainly from its own internal division, which turn voters like Mr. Smith away.
After Mrs. Thatcher, the leader who rates most highly in the polls is articulate, Kennedy-esque David Steel of the Liberal Party.
The older figure of Roy Jenkins, so-called ''prime minister designate'' of the alliance, has experience but a less clear image. Mr. Jenkins has been chancellor of the Exchequer and home secretary in Labour governments, as well as president of the European Commission. He has his admirers.
Sir Richard Attenborough, director of the film ''Gandhi,'' has just announced here that Gandhi himself ''would have voted for Roy Jenkins.'' Sir Roy certainly supports the alliance (just as jazz harmonica player Larry Adler and actress Maureen Lipman back Michael Foot and an array of comedians and others follow Mrs. Thatcher).
But Mr. Jenkins scores low in the polls. Tories lampoon his aristocratic accent and upper-class tastes. He has a tough fight to retain his own constituency of Glasgow Hillhead. He even had to abandon his campaign ''battle bus'' after its telephone, photocopier, and coffeemaker all stubbornly refused to work.
The ''first-past-the-post'' voting system here works against smaller parties. The SDP and the Liberals need 35 percent of the vote to score above 100 seats -- a feat that looks far beyond them at the moment.
They could end up with only about 20 seats, not even enough to hold the balance of power in a hung parliament.
Lacking the advantage of proportional representation, the Liberals hope voters will indulge in ''tactical voting.''
In seats with a narrow majority they hope that where Labour finished last in 1979 and a Tory won by a whisker over a Liberal, enough Labourites will switch to Liberal, therby defeating the Tory.
But most observers doubt that enough people will switch between Labour and the alliance to make much impact.
Several famous names may be beaten by recent boudary alterations that have changed their seats or forced them to seek new ones.
Among them: former Labour Education Minister Shirley Williams in Crosby; extreme left-winger Tony Benn in Bristol; and classical scholar and party individualist Enoch Powell in Northern Ireland.
Mr. Benn may squeak by unless a big Tory landslide elects his young opponent.
But Mr. Powell, a member of the Official Unionist Party, may be in real trouble. His South Down seat in Northern Ireland has been sliced by a quarter, excluding a strong pro-Unionist area around Banbridge.
With the loyalist, pro-British vote split between himself, a sub-postmaster running for the Democratic Unionists, and an accountant from the mainly Roman Catholic Social Democratic and Labour Party (SDLP), who is polling well, Mr. Powell's struggle is uphill all the way.
Whatever happens, the voter turnout here will be higher than for a United States presidential election.
In 1979 the British turnout was 76 percent of the electorate. It is expected to be at least as high this time -- in contrast to the November 1980 election in the US, in which 37 states failed to top 60 percent.
London will be crucial. More that one quarter of the city's 84 seats are vulnerable to a swing of 5 percent or so. As commercial TV news anchorman Alastair Burnet puts it, ''It could all be settled with a black taxi journey from the Palace of Westminister [home of the Commons] itself.''
The population has dropped 8 percent since 1971, as people have moved to the leafy avenues of the Home Counties where schools are better and crime is less.