The made-for-TV moment at the Liberal convention this weekend was Jean Chretien's celebrated encounter with the trombone, a little masterpiece of calculated, self-mocking charm. As one senior party official cackled, "that was worth five points [in the polls] on its own!" But a better picture of the leader's current frame of mind could be glimpsed in an off-hand crack earlier in the proceedings. Taking the stage with his cabinet to the theme from Men in Black, Chretien reminded his audience that he, too, sometimes wore the dark glasses favoured by the movie's title characters -- especially when giving his foes the "Shawinigan handshake." At which he motioned as if throttling an unemployed protester.

If the Prime Minister feels secure enough in his job to recall one of the public-relations catastrophes of his first term, the image also captured him in a moment of transition: from loveable little guy to aging tough guy. Chretien himself may not be going anywhere, but a subtle change in leadership is under way all the same. Essentially, he and Paul Martin are about to switch hats.

Much has been made of the obvious yearnings of the party grass-roots for a return to social activism. The tenor of the resolutions passed at the convention reflected this: more money for health care, early childhood nutrition programs, spousal benefits for homosexual couples, and so on. But as noteworthy was the firm back of the hand Chretien gave to most of these demands.

Time and again he lectured delegates -- even the odd cabinet minister -- on the importance of setting "priorities." Or in other words, no. There would be no return to the extravagant ways of the past, he vowed -- a promise the party accorded a less than rapturous response. In his closing address, the Prime Minister chose to emphasize tax cuts and debt reduction before investing in health care.

This is the inner tension of the Liberal party, and the key to its success to date. In a time of heightened public concern over the country's financial condition, it has been able to keep the right divided and itself in power by adopting a fiscal policy of sufficient rigor to attract the median voter. But while the party leadership leans to the centre-right, the party membership tilts resolutely to the left -- a fact of which prospective leadership candidates must be acutely aware.

On the face of it, a restive grass-roots would not bode well for Martin, Chretien's presumptive heir. For most of the life of this government, the Finance minister has been the standard bearer for the right wing of the party, imposing the sort of deep cuts in spending the Tories never dared. It was Chretien who reassured the party faithful as the defender of "traditional" -- i.e. 1970s -- Liberal values.

But now the roles are reversed. With Chretien around to mind the right, Martin will be free to spend the rest of the current mandate courting the party's left wing. Chretien, after all, has no need to curry favour with the grass-roots: his leadership is secure, and he is unlikely to stay on past the next election. From now on, then, it will be the prime minister who soothes the public -- and the markets -- with the dominant centre-right melody, while the leadership aspirants are performing their variations below.

This trick will probably work for a few more years, yet. Eventually, however, the left will grow tired of being patronized with empty rhetoric, and demand that the party deliver. If that happens before the next leadership vote, Martin's succession is anything but assured. Should the Liberals tilt too strongly back to the left, moreover, they risk giving new urgency to the "unite the right" movement, already energized by the imminent departure of Jean Charest as Conservative leader.

Assuming that Chretien wishes to reward his loyal lieutenant, therefore, the timing of his departure will be a delicate matter. He wants to give Martin enough time to refurbish his progressive credentials, without giving the party enough time to acquire a taste for something stronger. Complicating this calculation, of course, is the little matter of Quebec.

Contrary to received wisdom, Chretien is more than likely to stick around if it comes to a final confrontation with the separatist government of Quebec.

And, what is more, there is every reason why the party should wish to keep him on. The pressing need at that time will not be to win a referendum, but rather to manage the crisis surrounding it, since the Pequistes are highly unlikely to ask the sort of fair question the feds have insisted they must.

In such a crisis, Chretien's age and unpopularity in his home province, far from being a liability, would be an asset. Without the need to husband his political capital for future campaigns, Chretien would be free to perform the unpleasant but necessary tasks that might be required to forestall a unilateral secession : the "Shawinigan handshake," writ large. It would be his final service to the country, and a fitting one at that.