LONDON, JUNE 10 -- The leaders of Britain's main political parties headed to their home bases tonight after an exhausting 24-day campaign, each confidently predicting victory in Thursday's general election.

Final nationwide opinion polls indicated that Prime Minister Margaret Thatcher and her Conservative Party were headed for a third straight term, with a reduced but still healthy majority of seats in the 650-member Parliament.

Predicting a secure future of ever-greater prosperity under her government, and disaster with the Labor Party, Thatcher said "the choice at this election is clear. Indeed at no election in my lifetime has it been clearer."

But Labor leader Neil Kinnock, exuding confidence after what is widely considered to have been the best campaign performance of the race, insisted that Britain was seeing "the last days of Thatcherism."

David Owen and David Steel, of the third-party Alliance coalition of the Social Democratic and Liberal parties, brushed off their relatively poor showing in opinion surveys. They maintained that Britain's "thoughtful voters" would reject what they call the right and left extremes of Thatcher and Kinnock and give them a late surge of support.

The latest opinion poll, by Marplan, due out in The Guardian Thursday morning, gives the Conservatives 42 percent of the vote, with 35 percent for Labor and 21 percent for the Alliance. If those figures hold up, they will give Thatcher a majority of 40-60 seats, a position more in line with her first term than with the massive 144-seat majority she won in 1983.

Although the percentages conform closely with other polls conducted nationwide earlier this week, the Conservatives were known to be concerned today over more selective surveys in the so-called "marginal" Tory-held seats threatened by strong Labor votes. A survey of marginals released last night by the BBC indicated there is still a possibility of a hung Parliament, in which no party would have a working majority.

In that event, the party with the most seats, either Labor or the Conservatives, would have to try to form a government with outside support. The most likely coalition partner would be the Alliance, which has said its price would include extensive revisions of Britain's electoral system and more centrist defense and social policies.

Both of the main parties have rejected the idea of a coalition government, and most commentators here today still believed that anything but a Thatcher majority is extremely unlikely.

The campaign has yielded some significant surprises and portents for Britain's political future.

As one Labor analyst noted, Labor began with virtually no hope of victory and the possibility of a third straight defeat of such magnitude that questions were raised as to whether Kinnock could remain as its leader. Would Labor itself, portrayed by the Conservatives as ever more extremist and out of tune with British society, even be able to continue as one of the two leading parties here? There were widespread predictions of a major post-electoral realignment toward the center that would leave the moderate Alliance as the effective party of the left.

Instead, the Alliance bandwagon never seemed to take off. While Owen and Steel have been given substantial credit for waging the most issues-oriented, serious-minded campaign, their support appeared to slip steadily from the very first days.

Labor, on the other hand, began with a bang and rarely faltered. Concentrating on its strengths -- a promise to decrease unemployment and increase spending on the National Health Service -- it showed little of the insecurity and bickering that marked its 1983 effort. Instead, its performance was tight, professional and, in the words of one supporter, "ruthlessly organized and ruthless."

Instead of continuing its decline, the party appeared to recapture many of the traditional Labor supporters who had left it in despair in 1983.

Labor's campaign focused closely on Kinnock himself, and his leadership potential. As a result, party insiders this week claimed that Kinnock had won a major victory of sorts, even though he is unlikely to be Britain's next prime minister. "He has immeasurably strengthened his position inside the party," said one Labor analyst.

"His authority has increased enormously, not just because it was a good campaign but because it was his campaign. His people organized it."

To the extent that the electoral equation, as seen by public opinion polls, has changed during the course of the campaign, it has been in Labor's rise from the political basement to a respectable second place. While the Tories have not lost points from the 42-44 percent of the vote they started with, neither have they gained.

The image of Thatcher as uncaring, and her policies as harmful to Britain's underclass, appeared to touch a chord in many voters, even among supporters who share her view of Labor as disastrous for the country's future.