Logic would tell us that a left-of-centre party would prefer a centrist party to a right-of-centre party. But then, logic doesn't always apply to the NDP.

New Democrats hate Liberals. More than Conservatives. More than almost anyone else. They hate them for lacking principles. They hate them for occasionally wooing one of their more ambitious and pragmatic members - the Ujjal Dosanjhs of this world - and then rubbing it in their faces. And more than anything, they hate them for running just left enough in every election to cut into NDP votes, then governing further to the right.

That explains why New Democrat leaders spend most of their time during campaigns attacking Liberals, even when Conservatives are in the lead. And it's why we'll be spared a political realignment that would do a lot more harm than good.

If recent polls are to be believed, a contingent of the population would welcome a Liberal/NDP merger. And already, there are more than a few Liberals floating the idea. Whereas it took three successive defeats to unite the right, the Grits - unaccustomed to time on opposition benches - are already contemplating whether radical action is needed.

Should they lose the next election, those calls will intensify - especially in the likely scenario that the Liberals have a left-of-centre leader at the helm. They'll do the math, count how many ridings they'd have won if they had all those NDP votes as well and conclude that vote-splitting is keeping them from power.

Fortunately, the New Democrats' core won't care about being kept out of power, because - unlike old Progressive Conservatives - they have no expectation of power in the first place, and wouldn't know what to do with it if they somehow stumbled into it. And for that, we should count ourselves lucky.

Were the NDP and Liberals to merge, Canada would have a two-party system in nine of 10 provinces - and possibly Quebec as well, if the Tories struck a Mulroneyesque deal to bring soft nationalists on board and effectively wipe out the Bloc Quebecois. And if experience south of the border is any indication, that's something to avoid.

There are many things to admire about the United States, but its political culture is not among them. Particularly in the age of Fox News and the blogosphere, it's an Us vs. Them battle that's much less about policy debates than potshots - at once divisive and enormously stagnant.

In the 2004 presidential election, 47 of the 50 U.S. states went the same way as they did in 2000; meanwhile, a grand total of three House of Representatives incumbents were defeated outside Texas (where seats were redistributed). That's because, for most Americans, going Democrat or Republican isn't a choice they make every four years - it's a matter of identity, often one they're born into. So elections become less about convincing undecided voters than about the parties rallying their bases - hence the success that George W. Bush had in utilizing his opposition to gay marriage, which was barely even on the table in most states, to help his re-election.

In Canada, even with the NDP and the Bloc still kicking, there are signs we're moving in that direction. In many regions, support for one party is so entrenched that it seems unlikely to ever change. Parties are increasingly trying to motivate their bases by picking fights with regions that don't support them (the Liberals with Alberta; the Tories with Toronto). Throw in the more confrontational style adopted by the new government, augmented by the more adversarial brand of identity politics spreading outward from the blogosphere, and Us vs. Them is no longer just an American phenomenon.

So long as the NDP is around, it won't fully take root. The Liberals don't have enough support among either their centre-left or centre-right supporters alone to win elections, so they have to strike a balance. But if their only competition were the Conservatives, it would be easier for them to play to the hard-left than to try to win centre-right votes - leaving those to the Tories, and effectively leaving both parties simply playing to their respective bases.

The Tories already cheer the NDP's renewed existence, drawing votes from the Liberals as they do. The rest of us should as well. All that hatred of the Liberals is what preserves our political culture.