More incumbents have chosen not to run for re-election than in any campaign since the last majority government was dissolved in 2004 at least. With about one-fifth of formerly sitting Members of Parliament not putting their name on a ballot, the long list of reasons why the 2015 federal election campaign will be especially unpredictable gets longer.

Though conventional wisdom says a local candidate is only worth about five percentage points, there is no denying that certain incumbents carry more weight than others. Their retirement can turn what was once a safe riding for one party into a victory for another.

But the 2015 election may be won around the margins, where a few hundred votes in a handful of ridings could decide the outcome.

Lacking an incumbent can pose serious challenges for a party. Over the last two elections, parties were 17 per cent more likely to win a seat if they had an incumbent running than if their incumbent was not standing for re-election. That turned a party's re-election chances from better than three-in-four in a riding with an incumbent to just two-in-three in a seat without one.

The penalty for lacking an incumbent can be significant. All else being equal, parties without an incumbent have suffered a hit worth about seven per cent of what the party might have otherwise been expected to get in the riding. This is in consideration of how the wider region swung from the previous vote, and based on an analysis of more than 250 cases in recent elections.

Depending on the level of support an incumbent previously had in a given riding, that can represent anything from two to five points — seemingly aligning with the conventional wisdom concerning the value of a local candidate. And this is just the average performance. There are many individual cases where the loss of an incumbent had a much more profound impact (and, though fewer, some where the impact was negligible).

Lack of incumbents hurts Conservatives

Though the Tories have been hit particularly hard by the loss of some well-known incumbents like James Moore in British Columbia, John Baird in Ontario and Peter MacKay in Nova Scotia, the number of Conservative incumbents who have chosen not to run for re-election is not abnormally high. In fact, it is roughly proportional to the number of seats they won in 2011.

But the locations of those Conservatives not running for re-election could be problematic.

The vast majority of the New Democratic MPs not running again are in Quebec and British Columbia, where the party is leading comfortably in the polls. For the most part, the few Liberal MPs not re-offering are giving up seats the Liberals would be heavily favoured to win anyway.

By contrast, the Conservatives are missing a large number of incumbent MPs in British Columbia, Alberta, the Prairies, and Ontario — regions where they are facing many competitive races that they badly need to win. These departing MPs have opened up opportunities for the opposition parties.

A majority of the seats in which the Conservatives lack an incumbent (either through retirement or because the MP has opted to run in a different riding) are at risk of being lost to the New Democrats or the Liberals, according to seat-level projections. In all, that means almost 30 seats where the Conservatives could certainly use the added security an incumbent MP provides. By comparison, only a third of vacant Liberal ridings and a fifth of NDP seats without an incumbent are at risk of being lost to another party.

The polls continue to show a very close race. At current support levels, there is a fair chance that the New Democrats and Conservatives could finish within a handful of seats of one another. In those 30 to 40 seats without an incumbent that are at play, all three parties will be sorry to lose the two-to-five points advantage a familiar face can provide.