Andrew Leslie is just the latest Liberal MP to take a pass on the October election. But the only really unusual thing about the list of Liberal incumbents opting not to run again is that, by the standards of first-term governments, it's pretty short.

Leslie, a retired lieutenant-general in the Canadian Armed Forces who was touted as a star candidate when he was first elected for the riding of Orléans in 2015, brings to 13 the number of MPs elected under the Liberal banner who are not running again in the fall.

Another five former Liberal MPs now sitting as Independents have not yet confirmed their plans for October. They include former cabinet ministers Jane Philpott, Hunter Tootoo and Jody Wilson-Raybould. (Philpott and Wilson-Raybould were, of course, ejected from the Liberal caucus in the wake of the SNC-Lavalin affair.)

That brings to 18 the number of ridings where the Liberals won't have an incumbent on the ballot in October.

That might sound like a lot. It isn't.

Every governing party first elected to office in Canada with a majority government has seen at least 10 per cent of its caucus not run for re-election at the next opportunity.

For Trudeau's Liberals, the number of one-and-done MPs adds up to just seven per cent of the 184 seats the party won in 2015 (or just under 10 per cent, if the independents are added to the list).

Expect more MPs to bow out

An analysis of the data on the Library of Parliament website shows that first-term majority governments have seen an average of 22 per cent of their caucuses (defined as the number of seats won in the previous vote) decide not to run for re-election — about three times the Liberals' current rate of non-incumbency.

Looking only at first-term majority governments since 1925 — when the rate of total non-incumbency fell below 40 per cent, where it has stayed in every election since — the average rate is still 17 per cent.

Trudeau's Liberals would have to lose another 20 or so incumbents in order to meet the historical average.

The number of incumbents not running for re-election undoubtedly will increase between now and October. If you include the opposition parties, the percentage of incumbents choosing not to run again this year currently stands at about 23 per cent. Since 1925, the average for the entire House of Commons has been around 29 per cent. So it seems very likely that other MPs will announce they won't re-offer in the coming months.

But unless there's a surge in Liberal MPs heading to the exits soon, there's no indication in these numbers that the Liberals should worry about losing in the fall solely based on their lack of incumbents. (The polls give them reasons enough to worry about that.)

Opposition parties have more MPs not running again

Incumbents certainly do have value at the ballot box. Since Confederation, incumbents have been re-elected at a rate of about 76 per cent. And various studies have suggested that an incumbent candidate enjoys a bump of about five percentage points at the polls. That can be a significant advantage in a close election.

And it might worry the opposition parties now that, collectively, about 16 per cent of their MPs will not be running again — above the 14 per cent average for opposition parties prior to an election coming after a first majority term.

This is a rare situation in Canadian politics. In nearly four-fifths of all pre-election periods, the governing party loses more of its caucus to retirement than its opponents.

Not all incumbents are created equal, of course. The Liberals will be lacking incumbents in swing ridings in the Greater Toronto Area like Whitby, Oakville, Etobicoke Centre and Newmarket–Aurora. Orléans is a classic swing riding and the Liberals won't have incumbents in five of their 11 seats in Nova Scotia.

The Conservative incumbents not running for re-election, on the other hand, mostly hail from rural ridings that are unlikely to change colours.

More like Chrétien than Mulroney

Canada's last first-term majority government was Jean Chrétien's, from 1993 to 1997 (Stephen Harper's first majority in 2011 came after two minority terms in office). According to the Library of Parliament, just 19 Liberal MPs did not run for re-election in 1997 — 11 per cent of the 177 seats the party won in 1993. That appears to be the range in which Trudeau's Liberals could end up.

Brian Mulroney's Progressive Conservatives, at the end of their majority government's first term in 1988, saw 16 per cent of their incumbents decide not to run for re-election. Today's Liberals would need to see their number balloon to 29 MPs to match that score.

Both the Mulroney and Chrétien first-term governments were re-elected with majorities.

Defeated governments do tend to have higher rates of non-incumbency than re-elected ones. About 21 per cent of Harper's caucus didn't run for re-election in 2015. That was the highest rate for a governing party since 1993, when Kim Campbell's PCs were reduced to just two seats.

You have to go back to W.L. Mackenzie King's Liberals in 1945 to find the last time a governing party saw more than one-fifth of its caucus choose not to run again. King wasn't defeated in 1945 but his party did suffer significant seat losses.

On Wednesday, Ontario Liberal MP Andrew Leslie announced he was not seeking re-election this fall. (Adrian Wyld/Canadian Press)

It's hard to see a pattern in the numbers that would suggest the Liberals are in trouble because of a growing list of non-returnees. That's not to say it doesn't matter.

Again, not all incumbents are created equal: the mere fact that Philpott and Wilson-Raybould won't be carrying the Liberal banner in the fall may not have much impact on the results — but the reason why they're no longer Liberals almost certainly will.

The polls suggest the Liberals have an uphill climb ahead of them. That makes the relatively short list of incumbent MPs taking a pass on October somewhat surprising. It could indicate that the party doesn't think its chances are so slim after all.

If events between now and October suggest the party is wrong about that, expect to see that list grow. Swiftly.

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