Those who haven't worked in politics tend to assume it's at its ugliest during election campaigns. They're wrong. By far the most vicious, hard-nosed, bare-knuckle politics happens during leadership campaigns.

They're personal. They're divisive. They pit colleagues against colleagues, friends against friends, and leave wounds that take years to heal. Permanently embittering some participants, they drive others out of politics altogether.

Done properly, they are also the best thing that can happen to public discourse. Which is why all Liberals, even those who were encouraging him to run, owe a debt of gratitude to Frank McKenna for dropping out of their race before it even started.

It's been many, many years since one of Canada's major parties has had a robust leadership campaign featuring serious candidates debating serious ideas. But at least our conservative parties (in their myriad forms) and the NDP have come close. The Liberals have not had anything resembling a real race since Jean Chretien bested Paul Martin 16 years ago. And they're infinitely poorer for it -- a party lacking any defining vision or direction, because its members have not had the opportunity to choose one.

Had McKenna wound up the Liberal leader, he would have been a considerable upgrade over his predecessor. But his mere entry into the race would have diminished it. With the former New Brunswick premier and erswthile ambassador to Washington as the prohibitive favourite, most other qualified candidates would have stayed out.

For a time after McKenna dropped out, it seemed not so much an open field as an empty one. With John Manley, Brian Tobin and Allan Rock similarly signalling their lack of interest, the party briefly appeared to be turning into a punchline.

What has happened in the past few weeks, however, makes this the most exciting time for Liberals since they swept to power in 1993.

From across the country, a collection of promising candidates are revving their engines. All have their flaws. But the interest of a few in particular suggests the Liberals could be headed for that rare race in which smart people compete with other smart people on the basis of their ideas.

Political novice though Michael Ignatieff may be, it's rare for a party to have a single candidate of his intellectual calibre in the running. What is remarkable for the Liberals is that they'll have several others who can hold their own with him.

Since a somewhat disastrous tenure as Ontario premier, Bob Rae has established himself as one of Canada's leading centre-left voices -- a pragmatist of considerable depth. Until recently, his entry into the race seemed a long-shot. But under pressure from his brother, who ran Chretien's campaigns - and possibly from the former PM himself - it's starting to seem more likely.

When Stephane Dion was drawn into Chretien's Cabinet following the 1995 referendum, it was widely speculated he was too smart for politics. Instead, he achieved near-legendary status within his party as Intergovernmental Affairs Minister - the only imaginable federalist who could win battles with sovereigntists by sending them letters. Contrary to the popular belief that he's insufficiently partisan for leadership politics, Dion has been busy establishing a national organization.

That he was a Progressive Conservative little more than two years ago makes Scott Brison a controversial candidate. But he was the smartest, most ideas-driven contender when he ran for the Tory leadership in 2003 - a guy who loves talking about policy not just in the broad sense, like Martin, but also in terms of specifics. With McKenna sure to carry Atlantic Canada, the Nova Scotia MP never would have run; now, he's almost certain to bring his blend of social liberalism and fiscal conservatism to the table.

There are others, too, of considerable gravitas - notably former justice minister Martin Cauchon, who has proven willing to take bold positions and defend them eloquently. But put any three of Ignatieff, Rae, Dion and Brison in the same room, and you'll have the highest combined IQ of any leadership debate in modern Canadian history.

The Liberals are being handed a rare opportunity to use a leadership contest as a means to carve out a new identity. And if they take advantage of that, Canadians stand to witness an intellectual debate rarely seen at the federal level.

By the end of it, perhaps we'll all be inclined to send Frank McKenna a thank-you card.