MONTREAL

On a seat-by-seat basis, the Liberals would be the main winners of a successful electoral coalition with the NDP.

Based on the results of the 2008 election, a coalition running on a joint platform and a common slate of candidates could add 42 seats to the current NDP-Liberal total of 114 MPs, bringing it on the winning side of a governing majority.

The bulk of those gains would be Liberal ones and more than half of them would be registered in Central Canada.

In Ontario, a combination of the 2008 NDP and Liberal vote would result in the election of 15 more Liberal MPs.

In Quebec, such an arrangement would translate into nine more Liberal seats — almost doubling the party’s score in that province.

Under such an arrangement, the NDP would add only a handful of new seats to its take, mostly in Western Canada.

The gain for the New Democrats would be the guarantee of a direct role in government for the first time ever at the federal level.

This model has obvious limitations. It presumes that the votes from the last election are transferable from the NDP to the Liberals and vice-versa and that is certainly not true of every single individual vote. But polls do show that the NDP and the Liberals are more often than not each other’s second choice.

In logic, a voter who supported Stéphane Dion and his Green Shift in the last campaign is likely immune to the political charms of Stephen Harper’s Conservative party, as are, by definition, the bulk of NDP supporters

By tapping into a larger potential pool of support, Liberal and NDP incumbents would also largely be sheltered from whatever fallout could result from an electoral coalition between their two parties and from hostile Conservative takeovers.

On the positive side of the ledger, the model does not reflect the possibility that a significant number of Bloc Québécois and Green Party voters could decide to support a progressive coalition that stood a serious chance of unseating the Conservatives.

The results of the exercise mostly highlight just how difficult the status quo will make it for the Liberals to gain the two dozen or more seats they minimally need to be in contention for a governing role in the next Parliament, as well as how hard it would be for the NDP to vault to a second-place finish. They have too many occasions to shoot each other in the foot.

For example, the list of Quebec ridings where the Liberal and the NDP basically ensure each other’s defeat to the Bloc Québécois is almost exclusively made up of ridings that voted No to sovereignty in the 1995 referendum. The split in the progressive federalist vote has allowed them to fall into the BQ’s lap.

Last weekend, Liberal Leader Michael Ignatieff ruled out a campaign alliance with the NDP but left the door open to a joint parliamentary arrangement if the election results warrant reaching out to Jack Layton to deny the Conservatives a third term.

In so doing Ignatieff basically empowered his opponents at his own expense.

He has given NDP supporters a bigger incentive to stick with Layton in the next campaign in the reasonable hope, given the current polls, that their votes would give him a stronger hand in an eventual post-election negotiation with the Liberals.

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And he has allowed Conservative strategists to fill in the blanks of his post-election coalition musings in ways that fit their agenda.

As things stand now, Ignatieff is poised to go in the next campaign bearing all the real and imagined risks of a potential coalition with Layton but enjoying none of its probable advantages.

Chantal Hébert is a national affairs writer. Her column appears Monday, Wednesday and Friday.