London — If one took seriously the euphoria of the winners of the latest parliamentary by-election, one might think that Britain was about to become a one-party state.
Some supporters of the recently formed alliance of Liberals and Social Democrats now seem to believe their joint political movement can sweep country.
True, the alliance did achieve a big win Oct. 22 in formerly Conservative-held Croydon North West, on the fringes of London. That ''by-election of the generation'' (or ''of the century'' - you name it, some alliance backers have probably said it) will keep the alliance bandwagon rolling.
But opinions vary widely on whether the by-election has really broken, or merely cracked, the mold of the two-party political system that has produced alternating Conservative and Labour governments here since before World War II.
What happened is that a Liberal, William Pitt, after doing progressively worse in three past bids for the seat, suddenly did 29 percent better when in harness with the Social Democratic Party (SDP) to win the first-ever election for the alliance. He took votes almost equally from the Conservatives and Labour.
In an earlier by-election at Warrington, the first-ever SDP candidate to run, Roy Jenkins, had come devastatingly close to winning that ''safe'' Labour seat. In a sense, considering the huge odds, Mr. Jenkins did even better than Mr. Pitt.
But Jenkins is a former Cabinet minister, a political star. Mr. Pitt is just Mr. Pitt, a local politician said to have the charisma of a surprised teddy bear. Hence the uproar.
By-elections, nonetheless, are a notoriously unreliable guide to long-term political trends. They cannot really give an opportunity for much more than protest. And Croydon opinion polls showed 60 percent of voters were indeed protesting against Conservative government policies or Labour opposition ineptitude. Only 25 percent were voting positively for the alliance.
And since this was a by-election, there was no question of the alliance being voted into office. Liberals have in past years won even more spectacular victories than this in similar dormitory suburbs of London without being able to capitalize on them at the next general election.
But why did Labour, while losing slightly less support than the Tories, still slide into an ignominious third place in the Croydon by-election? Halfway through a term of an unpopular Tory government, Labour should have done well. In fact, it has not gained a seat in a by-election for 10 years.
Labour leader Michael Foot thinks the ''overwhelming'' cause of Labour's Croydon defeat was the ''distractions and dissensions'' in his party in the past year.
But there was another, London-related, factor that made the by-election atypical. At first, new Parliament member Pitt said on election night that he had won because the alliance had ''caught the imagination of the British people.'' The morning after, he said that Ken Livingstone, Labour leader of the Greater London Council - the big local authority - had been ''directly responsible.''
Door-to-door canvassers during the campaign found that electors resented Mr. Livingstone for policies that touched their pockets in a way they didn't like. He had also upset people for saying after the first recent London bombing that the illegal Irish Republican Army, which claimed responsibility, was not ''just criminals.'' Although Mr. Livingston is not one of Labour's national parliamentary leaders, the party suffered guilt by association.
The enormous effort put into the campaign by the alliance also made it atypical. In a general election, alliance campaigners could not possibly visit every house in every constituency five times.
So the next general election result is not likely to be simply Croydon writ large across the nation.
Nevertheless, the alliance's gains may well have an impact on national policies. Another alliance star, Social Democrat Shirley Williams, hopes to overcome a Tory majority of more than 19,000 to win a forthcoming by-election at Crosby. The Tories' loss in Croydon may not be quite bad enough to persuade Prime Minister Margaret Thatcher to change her policies. But if the Tories lose Crosby, too, it could be a different story.