TIME MAGAZINE HAS made it official. Under the screaming headline "Canada Changes Course" the smiling visage of our new conservative Prime Minister adorns the edition of September 17th. Brian Mulroney now takes his place along with Cheryl Tiegs, various corporate worthies and innumerable portraits of Chiang Kai Sheik as Time's galaxy of cover personalities.

But before the editors of Time, the Wall Street Journal, and National Review wax too eloquent about the significance of Canada's recent election they should ponder this: the September 4th result was undoubtedly the Conservative Party's greatest electoral victory. It may also turn out to be Canadian conservatism's greatest single defeat.

Understanding that riddle is the key to comprehending the dynamics of the September 4th contest and in particular explains the calamity that befell the Liberal Party, Canada's majority party since the turn of the century. To put it simply, the Liberal Party was no longer viewed by Canadians as the sole or even the main defender of liberal values.

The sweeping success of Mulroney appears on the surface to conform to a pattern already evident in Britain and the United States, First Prime Minister Margaret Thatcher, then President Reagan, and now Brian Mulroney have won stunning victories for conservatism.

But the origins of their respective victories diverge radically. Both Thatcher and Reagan offered a bold challenge to the liberal consensus governing their countries' societies. They set out to change the underlying assumptions of social democracy. Their policies were put forward with clarity, and each received a mandate for a specific course of action.

Mulroney has fashioned a different kind of victory. Rather than challenging liberalism, he embraced it.

Since the turn of the century the dominance of the Liberal Party has been based on three fundamental pillars:

*The reconciliation of French and English Canada.

*The extension of social security to the disadvantaged.

*The creation of wealth and economic opportunity for Canadians wanting to get ahead.

MULRONEY TOOK DEAD AIM at each of these central propositions. He made the strategic decision that his right wing supporters had nowhere else to go, thereby giving him the luxury of fishing in liberal waters.

He opposed his own Conservative Party in Manitoba on the issue of Francophone language rights. He supported the Trudeau government's position on banning extrabilling and health care. And above all, he replace the traditional Tory theme of "short term pain for long term gain" with a song of his own--"Let the Good Times Roll."

Virtually any group that moved was promised a tax break; billions of dollars, Mr. Mulroney said, could be found to reduce the deficit without cuts in social services. Followers of Ronald Reagan should be flattered by the obvious acute observation Mr. Mulroney has made of the President's political success. Like Mr. Reagan's deficit policy, Mr. Mulroney's leap towards the center turned conservative dogma on its ear but it made an impact in the ballot box.

Where does this leave the Liberal Party? The magnitude of its defeat cannot be minimized. Under Pierre Trudeau the Liberal Party had fashioned an electoral coalition made up of women, young people, ethnic minorities, and French-speaking Canadians. Each of the groups spurned John Turner, Mr. Trudeau's successor. The critical electoral fortress of Quebec has been lost. The socialist New Democrat Party regained many supporters that Mr. Trudeau had successfully weaned away. Despite Mr. Turner's courageous decision to seek and win a seat in Western Canada--the one part of the country that remained impervious to Mr. Trudeau's charms--the beachhead is small.

But amid all the doom and gloom and the painful reassessment that must inevitably follow such a setback, Liberals can take heart from one basic fact. Though we were found wanting, our philosophy was not.