Losing an incumbent MP hurts in an election campaign — but losing some incumbents hurts more than losing others. That's something the Liberals and New Democrats could discover to their sorrow in October.

The Liberal roster is heading toward the fall vote in relatively good shape, with a smaller share of the party's caucus not running for re-election than is usually the case for a majority government, even a first-termer. Relatively speaking, the Conservatives and NDP have lost much bigger chunks of their teams than the Liberals have.

But the Conservative vacancies are mostly in ridings where the party is all but guaranteed to win. For the Liberals, nearly half of the vacancies are in ridings their party could struggle to hold with new names on the ballot.

For the the New Democrats, it's even worse: almost all of their vacancies are in ridings where the NDP can expect a difficult fight.

Including Scott Brison and Nicola Di Iorio — former Liberal MPs who stepped down earlier this year after the cut-off date for calling a byelection to fill the vacancies — there are (so far) 45 incumbents not running for re-election this year. And two former Conservative MPs (Mark Warawa and Deepak Obhrai) passed away recently.

At 14 per cent of the House, that's on par with historical patterns for majority governments. On average, 15 per cent of MPs have opted not to run for re-election at the end of a majority term in office over the last century.

Of those 47, 17 are Liberals, 15 are Conservatives and 11 are New Democrats. There are also four Independent MPs not running for re-election. They include two former Liberals, one former Conservative and one former New Democrat.

Those are pretty even numbers — except when you consider the size of each party's respective caucus. Including the Independents, seats without incumbents represent 29 per cent of the NDP caucus, 17 per cent of the Conservative roster and just 11 per cent of the Liberal team.

How many of these seats could be lost to the parties holding them now?

NDP down incumbents where seats are at risk

Out of the three parties, the NDP is most in need of incumbents — the party is slumping in the polls and trails its rivals in the number of nominated candidates.

Using the Poll Tracker's seat projection model, we can estimate how many of these incumbent-less seats could be vulnerable. For the New Democrats, it could be all of them.

NDP Leader Jagmeet Singh poses for a photograph with a woman while on a tour of the Montreal borough of Outremont Saturday, December 22, 2018. The NDP's seats in Quebec are under threat - especially in ridings without an incumbent. (Graham Hughes/The Canadian Press)

The model estimates that of the 12 ridings where the NDP will not have an incumbent MP on the ballot, three of them are potentially vulnerable — meaning the projection model gives the NDP the best chance of winning, but other parties remain competitive.

In another three, the New Democrats are estimated to be trailing but still competitive themselves. In the remaining six, the NDP is projected to lose.

The NDP is particularly threatened in Quebec, where it's polling at roughly 11 per cent. That's a drop of about 14 points from where the party stood in the province in 2015. That drop alone theoretically puts every seat in the province up for grabs. Lacking incumbents just compounds the challenge for the party in some of these seats.

Conservatives lacking incumbents in safe seats

The Conservatives, despite having lost a greater share of their incumbents than the Liberals, are unlikely to suffer much.

Twelve of the Conservatives' 16 seats without an incumbent on the ballot are considered 'safe' seats. They're largely rural ridings that the Conservatives simply won't lose — seats like Battle River–Crowfoot and Yellowhead in Alberta, or Cypress Hills–Grasslands in Saskatchewan. The average Conservative margin of victory in these three ridings was over 61 percentage points.

There are a few no-incumbent ridings held by the Conservatives that look vulnerable in Ontario, where the unpopularity of Premier Doug Ford's Progressive Conservative government has dragged down the federal party's numbers. But any seat the Conservatives won in 2015 is unlikely to be a vulnerable seat to begin with: defeated parties tend to be reduced to their strongholds.

Liberals could use familiar faces in some ridings

The Liberal list of ridings without incumbents (including Brampton East and Calgary Skyview, where the MPs are now sitting as Independents and have not yet announced their plans for the fall) is a mix of safe and swing seats.

A dozen of them are considered safe — seats in Toronto, Atlantic Canada and Quebec. In another seven, the Liberals are either estimated to be leading-but-vulnerable or trailing-but-competitive. There are two in which the Liberals are not considered to be in the running.

It suggests that the Liberals should have reasonable hopes of holding on to as many as two-thirds of the seats in which they lack incumbents — which wouldn't be a bad performance, considering that the historical re-election rate of incumbents is about 76 per cent.

But a few of the Liberals' incumbent losses may hurt more than the raw numbers suggest.

Farewell to Nova Scotia

Among the list of 12 safe seats for the Liberals are Nova Scotia ridings like Cape Breton–Canso, Kings–Hants and Sydney–Victoria. The Liberals won these seats by an average margin of 57 points, thanks in part to the local popularity of Rodger Cuzner, Scott Brison and Mark Eyking. Normally, ridings won by such gargantuan margins would be considered invulnerable.

Not these, however. The Liberals have some concerns about holding on to these seats — and the Conservatives have some hopes of winning them.

Atlantic Canada is one of the places in Canada where a loss of incumbents hurts the most. Over the last few provincial and federal elections, Atlantic Canadian ridings were over-represented on a list of seats where the biggest swings in support coincided with the loss of an incumbent candidate.

So if these Nova Scotian seats are moved into the vulnerable category, it means a majority of the seats where the Liberals lack an incumbent could be up for grabs.

In what could be a close election (the Poll Tracker puts the Conservatives and Liberals neck-and-neck), these seats might prove to be decisive. For political parties, having a lot of incumbents helps — but having them in the right places can help even more.