All over Britain small bands of people are being mobilized, ready for action at a moment's notice. The party faithful have been put on the alert.

Constituency workers are the backbone of any political party's election campaigning, and as the conviction grows that Prime Minister Margaret Thatcher will call a general election next year, so activist members of the Conservative, Labour and Social Democratic/Liberal Alliance parties are gearing themselves up.In Britain, the election process is very different from that in the United States. There is no fixed term of office for the government, just a maximum of five years for any one Parliament. The prime minister can go to the country whenever he or she wants, often choosing the moment when conditions seem most favorable for the party in power.

In 1974, Labour Prime Minister Harold Wilson called a general election in October, just eight months after winning the February parliamentary elections, in an effort to increase his majority in the House of Commons. But the Labour Party then waited almost the full five-year term before holding the next general election, the one that saw Mrs. Thatcher swept to power.

Well into her second term of office, the prime minister is having to consider the optimal time to go to the polls. In theory, she could wait until June 1988, but there are a growing number of signs that it will be sometime next year. The reshuffle of government ministers a couple of weeks ago was clearly designed to promote those members of Parliament seen as communicators.

With the exception of Mrs. Thatcher herself, most Cabinet ministers are not well known to the electorate. Recent polls suggest many people in Britain could not name the foreign secretary, Sir Geoffrey Howe, despite the fact that he has been a senior member of the government since 1979. Most of the better known or more flamboyant ministers of recent years, such as Michael Heseltine, Cecil Parkinson or Lord Carrington, have, for a variety of reasons, resigned.

While leaving her present Cabinet intact, Mrs. Thatcher brought a number of new faces into the middle and lower ranks of government. Her goal in the reshuffling was to appoint those she feels will do a better task of explaining and promoting government policy.

The surprise announcement that British Airways will be privatized early next year was also seen as an indication the government is planning an early general election. Mrs. Thatcher has been pressing for the sale of BA since she first took office, and has been disappointed by the many delays. At last the way is clear, and the government seems determined to complete a share flotation before an election is held.

From now on, virtually every government action will be scrutinized for clues as to possible timing. At the moment, two possible windows are deemed likely - either in May-June soon after the budget, or in September-October after the summer recess. The problem for the opposition parties is that Mrs. Thatcher need give little more than two weeks notice.

In practice, an election campaign normally runs for three weeks. While the British system is clearly preferable for voters - who are not exposed to months and months of electioneering that precede a U.S. presidential election - the system is nevertheless tough on party workers, who must respond instantly once the prime minister decides to ask the sovereign to dissolve Parliament.

The differences between the two systems do not stop there. Britain is divided into 650 constituencies, and electors vote only for one local candidate. There is no direct voting for the head of state, as there is in the United States. The political party that returns the most MPs to the House of Commons is then invited by the queen to form a government.

Each party has its own procedure for choosing the leader who, in the event of victory, would be appointed prime minister. Because voters only elect their own MPs, a general election does not have to be called should the prime minister resign or die in office.

There is no equivalent of an elected vice president in Britain.

When Harold Wilson resigned in 1976, his successor, James Callaghan, was chosen by the Labour Party. And should the Conservative Party decide that Margaret Thatcher was an electoral liability, it could replace her with a new leader.

At this stage, it is impossible to guess who might occupy 10 Downing Street in a year's time. The Conservative Party and Margaret Thatcher appears out of favor right now but, as the Falklands War showed, political fortunes can alter very swiftly. Although the Conservative Party never exploited the Falklands victory in their last election campaign, the Falklands factor undoubtedly contributed to Mrs. Thatcher's huge parliamentary majority in the 1983 general election.

This time round, however, Labour seems better prepared to wrest control of the government from the Conservatives. Labour's standing with the electorate had sagged badly under the leadership of Michael Foot. Mr. Foot was seen by many as an old-fashioned type socialist whose methods were out of step with current political thinking.

New leader Neil Kinnock has restored the party's reputation, however, offering a more middle-of-the-road platform and a more modern image.

The next election may also see the emergence of the SDP/Liberal Party Alliance as a powerful political force. In the past, however, the party has fallen victim to Britain's "first past the post" election system, in which only the candidate with the most votes is elected. Although the Alliance only won 23 parliamentary seats in the last election compared with Labour's 209, it nevertheless gained 25 percent of the votes cast, only just behind Labour's 27 percent. Needless to say, the party is a keen advocate of proportional representation.

But despite this handicap, the Alliance could find itself holding the balance of power - and perhaps, being asked to form a government - if as some commentators are forecasting, there is a hung Parliament after the next election, with no one party gaining an overall majority.

For the past 150 years, a mainly two-party system has existed in Britain, with the right-wing Conservative Party or the left-wing Labour Party dominating Parliament for most of this century. The emergence of the SDP/ Liberal Alliance over the past few years into an important political force could dramatically change the face of British politics.

Right now, though, it is Mrs. Thatcher who has the upper hand, able to choose the moment when the polls look most favorable to try and win a third term of office. The other party leaders have put out an election alert to their followers, but can do no more until the prime minister makes her move.

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