Has Britain's Social Democrat-Liberal Party alliance bandwagon achieved the momentum that will make it a real political force in the land? Shirley Williams' victory in a by-election for the Crosby parliamentary seat suggest it has.
Mrs. Williams, an original member of the Social Democrat ''gang of four'', converted a town with a 19,000 Conservative constituency into a 5,000 majority for herself. In the process she crushed her Tory opponent, humiliated the Labour Party standardbearer, and sent political statisticians hurrying to their computers. She has now become the first Social Democrat actually to be elected to Parliament.
Mrs. Williams started well behind in Crosby, a traditionally Conservative area. But 10 days before polling day it was already apparent that she was the probable victor.
Her personal appeal as a sincere and hard-working candidate was one reason for her good performance.
But perhaps even more of an influence was the Social Democrat-Liberal option. Unimpressed by Prime Minister Margaret Thatcher's handling of the economy and averse to backing an opposition Labour Party in disarray, thousands of voters chose the ''third force.''
The Crosby result follows a victory by the alliance six weeks ago at Croydon near London. Earlier, another member of the ''gang'' - Roy Jenkins - very nearly won a seat against heavy odds in Warrington.
The next time a winnable constituency is available, Mr. Jenkins is expected to fight for and gain it.
Tory and Labour luminaries, licking their party political wounds soon after Mrs. Williams (known to admirers as ''Shirl the Girl'') learned of her success, claimed that by-election upsets do not necessarily indicate what will happen at general elections.
One Conservative central office worker said: ''Eight or so years ago the Liberals had a series of successes at by-elections, but later on, the seats returned to their former occupants.''
Mrs. Williams rejected this analysis as did Mr. Jenkins, now the only ''gang'' member without a parliamentary seat. Both spoke in terms of a political earthquake caused by widespread disenchantment with the old two-party system.
David Steel, the Liberal Party leader, endorsed their view, predicting that at the next election the alliance would form the next government.
Such forecasts do not gain acceptance with election experts who point out that there was much tactical voting at Croyden and Crosby, with former Labour supporters switching to the alliance to get the Conservatives out.
Also there are many parliamentary seats where Labour or Conservative majorities are so huge that they are virtually unassailable.
Nonetheless even cautious election experts do accept another possibility - that the Social Democrat-Liberal alliance will do well enough to hold a balance of power after the next general election.
This calculation is based on the extent to which many former Labour and Tory voters will cast their ballot for the alliance.
It is here that the real earthquake potential of the alliance may be most felt. For if Liberals and Social Democrats form a big enough wedge in the next House of Commons they could force electoral reform on whichever of the parties forms the government.
Both elements in the alliance want Britain to abandon ''the first past the post'' system of counting votes and adopt proportional representation instead. If that happened, the electoral experts are saying, Social Democrats and Liberals would be better placed to exploit their voting strength in general elections.
One straw in the wind, suggesting that the political map of Britain is already changing drastically, was provided by the former Conservative Prime Minister Edward Heath.
Within hours of Mrs. Williams' success he let it be known that ''under certain circumstances'' he would join a government brought to office in the wake of a Liberal-SDP victory.