Bathed in bright lights for the television cameras, nearly 1,000 people overflowed a high school auditorium here in Scotland's largest city last night to hear Roy Jenkins, the standard-bearer of Britain's year-old Social Democratic Party.

They were told they might decide the country's political direction for years ahead when they vote Thursday to fill a vacant parliamentary seat in their Western Glasgow constituency of Hillhead.

In his final appeal in what has become Britain's most important election campaign since Prime Minister Margaret Thatcher's Conservatives came to power nearly three years ago, Jenkins said they could put "the stamp of Hillhead on the whole future of Britain" by helping him return to Parliament and become the leader of the centrist political alliance of his Social Democrats and the Liberal Party.

With Liberal leader David Steel, Scotland's most popular politician, at his side, Jenkins warned that at stake was the future of the alliance as a "moderate" alternative to the current "monetarism" of the Conservatives under Thatcher on the right and the "Marxism" of the Labor Party on the left.

Jenkins, 61, served in several senior Cabinet posts and as deputy leader of the Labor Party before resigning from Parliament to serve four years as president of the Common Market. On returning to British politics last year, he joined three other former Labor Cabinet ministers, Shirley Williams, David Owen and Bill Rogers, in forming the Social Democratic Party and linking it with the Liberals.

Derailing Jenkins and the Social Democratic-Liberal alliance has been the top priority of the Conservative and Labor candidates and their supporters race that up to now has been too close to call. Three opinion polls to be published in British newspapers election day show Jenkins to have taken the lead, although the other two major candidates also had been put in front in recent days in others of the many polls.

The unusually diverse constituency of just 40,000 voters mixes middle- and upper-class voters in Victorian mansions and villas on the hillsides with low-income and unemployed workers in run-down public housing in the industrial valley along the Clyde River. Glasgow University faculty members and young professionals and business executives give Hillhead the highest educational level of any of Britain's 635 parliamentary constituencies, but it also has some of the country's worst and most overcrowded housing.

This disparity has been stressed by the Labor candidate, 34-year-old Glasgow community worker David Wiseman, who has the support of his party's left and right wings in a rare show of unity. If Wiseman succeeds in joining the Labor members representing the rest of Glasgow in Parliament, it would demonstrate that Labor has been able--despite its internal feuding and defections to the Social Democrats--to retain much of its traditional base in Scotland, Wales and northern England, where the recession has hurt most.

The Conservative candidate, 31-year-old lawyer Gerry Malone, has been helped by campaign appearances from members of Thatcher's Cabinet, a written message of support from the prime minister and surprise approval by her penny-pinching government for a $50 million exhibition center to be built on abandoned dockland next to Hillhead.

Although Hillhead had been a safe Conservative enclave for 64 years before the death earlier this year of the incumbent, Sir Thomas Galbraith, a victory by Malone would rate as a major upset at a time unemployment is 12 percent nationally and much higher in Glasgow.

If he won, it would cap a recent comeback by Thatcher and the Conservatives in public opinion polls after its popularity plunged during Britain's worst recession in a half century. Earlier this month, one leading poll put the Conservatives ahead of the other parties in voter support for the first time since 1979.

Analysts attributed the resurgence to the Social Democrats' decline and apparently renewed confidence by Conservative supporters in relatively prosperous southern England that the long-expected economic recovery is imminent despite high unemployment levels elsewhere in the country.

A Conservative victory here also would weaken critics of Thatcher's economic policies inside her Cabinet and on the Conservative back-benches in Parliament, whom she has successfully outmaneuvered in recent months to strengthen her control of the government.

But the Conservatives' primary aim, in the words of one Thatcher aide, is to "bust Jenkins" in Hillhead because they still see the Social Democratic-Liberal alliance as their primary political threat. "A loss by the Social Democrats in Hillhead," said one senior member of Thatcher's Cabinet, "would be crippling to them."

A Jenkins win, on the other hand, would climax a series of special election successes for the Social Democrats and Liberals since Jenkins himself fell just short of achieving an upset in a safe Labor constituency in northern England last year. It also would be expected to reverse the allied parties' recent decline in national public opinion polls from a peak last year of declared support by two of every five British voters.

Williams, who campaigned with Jenkins here today, told a group of American reporters in London earlier this week that the Hillhead election is "absolutely critical" for the alliance and her party.

"There isn't any doubt if we were to lose it would have a substantial effect on the party's future," she said. "For any third political force in a traditionally two-party political system, the crucial question is credibility. And these by-elections have been the key to our credibility."

The race has been further complicated by the effective campaigning and strong showing in opinion polls of a Scottish Nationalist Party candidate, 45-year-old veterinarian George Leslie, and an assortment of minor candidates, including one promoting disarmament and another opposing the visit this June of Pope John Paul II to Britain, including a stop in Glasgow.

Nuclear disarmament has emerged as an important issue, with many voters expressing opposition to the Thatcher government's decision to purchase the U.S.-developed Trident submarine-launched nuclear missile system.

Malone has supported Thatcher's decision, Labor's Wiseman has advocated a unilateral ban on all nuclear weapons in Britain, and Jenkins has argued against the Trident deal as unnecessary and too expensive for Britain.