THREE'S a crowd in American politics. But Britain's third party could help shape the course of the government well into the next century.
The nation's main opposition party, Labour, may join an alliance with the Liberal Democrats, the nation's third-largest party, to keep the ruling Conservatives out of power for at least 10 years.
If put into effect, the concept of ''partnership politics'' at the core of the plan would transform voting patterns in Britain.
Senior Labour and Liberal Democrat (Lib Dem) figures were reported to have already begun informal talks about a post-election parliamentary partnership between their two parties.
The Conservative Party is widely expected to lose general elections due by next spring, but the partnership would help keep the Conservatives out of power in the next elections five years later as well.
Tony Blair, the Labour leader, has given a cautious welcome to a partnership initiative launched Jan. 22 by Paddy Ashdown, his Liberal Democrat counterpart.
Mr. Ashdown's centrist party currently holds 25 House of Commons seats. It has usually tried to distance itself from both Labour and the Conservatives. But Ashdown made it clear in a widely reported speech that he is seeking a deal with Mr. Blair to cooperate on legislation to modernize Britain.
But he made his offer conditional on Labour accepting electoral reform, including a new voting system to be put to the people in a referendum.
Labour Party officials say privately that Blair wants to explore Ashdown's partnership plan in greater detail and is attracted by a deal that would promise the two parties a working parliamentary majority over the Conservatives for at least a decade.
In the last two or three years, the Liberal Democrats have won a string of by-election victories in constituencies that normally vote Conservative, and the party's electoral appeal will be a powerful bargaining chip in Ashdown's dealings with Blair.
Opinion polls have shown that in several areas of Britain, voters fed up with the Conservatives would rather vote Lib Dem than support Labour. The defection to the Lib Dems from government ranks earlier in January of Emma Nicholson, a Conservative member of Parliament, underlined the attractions Ashdown's party has for dissatisfied Conservatives.
Ashdown's partnership politics agenda does not call for a formal coalition with Labour - he admits publicly that some sections of his party would think that would be going too far.
Instead, he is proposing cooperation on key sectors of policy. He pinpointed education, welfare and health reform, the environment, and policy toward Europe as areas where Labour and the Lib Dems could work together.
If Blair and Ashdown reach agreement on a post-election deal, Labour will be able to count on Lib Dem strength in key areas of the country, such as southwest England, preventing the Conservatives from winning seats.
But the price Ashdown will ask for Blair's agreement to a deal may not be to the taste of many Labour activists. He wants the Labour leader to accept the need for proportional representation (PR) in the allocation of House of Commons seats.
PR systems vary from country to country, but the general aim is to avoid a ''first past the post'' outcome in elections.
At present, if there are several candidates for one seat, and the vote is split among them, it is possible for a candidate to win with as little as one-third of the vote. All the other candidates lose.
Under PR, seats in Parliament would be distributed according to the proportion of votes gained by each party in each constituency and nationwide. This would almost certainly greatly increase the number of Lib Dem MPs in the House of Commons.
The Conservatives lean against PR, although it operates in many conservatively led European countries such as Germany.
In terms of his own personality, Ashdown's backing could help a Labour government. He is a charismatic campaigner with a flair for oratory.