London — The hottest political topic across Britain today is something called ''the Falklands factor.''
The Conservative government wants to hang on to it. Opposition parties dismiss it as temporary. The US is studying it for possible effects on British policies abroad.
But no one can tell just yet what its long-term impact might be.
In the short term, the war in the South Atlantic has had spectacular effect on British politics. The impact reads:
Prime Minister Margaret Thatcher -- way up.
Opposition Labour leader Michael Foot -- way down.
Liberal party leader David Steel -- about the same.
Social Democratic leader in the House of Commons David Owen -- up.
Aspiring Social Democratic leader nationwide Roy Jenkins -- down.
Other parties and leaders -- trailing.
Yet in politics, reality is rarely what is seems to be, and short-term gains today can evaporate or turn into losses in the longer view.
The big unanswered question is whether the enhanced image and popularity that Mrs. Thatcher has won by regaining the Falklands can carry over until the next general election, which most experts believe she will call in the autumn of 1983 .
The United States has a very large stake in the answer. Victory for Mrs. Thatcher would keep Britain firmly in NATO and in possession of nuclear weapons, whereas Labour is committed to nuclear disarmament, and is split on NATO membership. The Social Democrats would keep US cruise missiles which Mrs. Thatcher has already agreed to take as part of the NATO response to Soviet SS-20 missiles pointed at Europe but Labour would reject them. Mrs Thatcher is buying Trident nuclear missiles for new British submarines for at least (STR)6 billion ($10.5 billion) whereas both Labour and the Social Democrats would reverse the decision.
Right now, the ''Falklands factor'' shows up clearly in by-elections and in public opinion polls.
On the eve of the Argentine surrender in mid-June, a dramatic 52 percent of 1 ,075 adults questioned all over Britain in a major poll said it would vote for the government if a general election was held today.
Only 29 percent chose Labour, and 17 percent the Liberal-Social Democratic alliance.
Meanwhile, a Gallup poll the week before measured the sharp rise in Mrs. Thatcher's popularity since the task force set sail April 4.
In March, Gallup showed the Conservative Party (Tories) holding just under 32 percent of popular support and Labour 33. But by June, the government had rocketed to 45 percent and Labour had slumped to 25.
With Tories up and Labour down, the Social Democrats and the Liberals also found themselves struggling, mainly because public attention had shifted to the war.
The Social Democrats dropped 41/2 percent to 15 percent, and the Liberals 1 to 10 percent.
On top of the poll results, the war helped the government reverse the usual rule of British politics, which says that the party in power loses seats at mid-term. Tory candidates won by-elections in the London suburb of Mitcham and Morden June 3 (taking the seat from a Social Democrat and holding steady at 431/ 2 percent of the vote) and retained the prosperous London surburb area of Beaconsfield May 27, also with an undiminished share.
Tory party Chairman Cecil Parkinson is delighted. As might be expected, he tries to attribute success to non-Falklands issues as well:
''The trend towards us,'' he says, ''has been established for several months. In the last few weeks there has been a steady flow of economic good news which has inevitably been overshadowed by other events.''
Tory strategists are even more pleased that Labour is doing so poorly.
In five successive by-elections since mid 1981, Labour has dropped back behind Tories, Social Democrats and Liberals.