Both Justin Trudeau and Pierre Poilievre will face a stiff test in today’s federal byelection in Mississauga–Lakeshore. But as the party with something to lose, the Liberals are the ones on the hotseat tonight — as the governing party always is when a byelection comes around.

This byelection is the first one to be held since the 2021 election. It’s taking place in an important battleground, the winner of which usually gets to form the government. So, there’s plenty at stake.

Join me to watch the results as they come in! I’ll be providing live analysis and answering questions on YouTube, starting at 8:30 PM ET tonight. You’ll be able to follow along with me here:

This has come about due to the resignation of Sven Spengemann, who was first elected as Mississauga–Lakeshore’s Liberal MP in 2015. He was re-elected in 2019 and 2021 but stepped away from his seat earlier this year after taking a job with the United Nations.

Spengemann took 44.9% of the vote in 2021, defeating Conservative candidate Michael Ras, who captured 38.7% of the vote. The NDP finished well back in third with 9.8%, followed by the People’s Party candidate at 4.2% and the Greens at 2.3%.

Mississauga–Lakeshore is a key test for both the Liberals and Conservatives because of what it represents.

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For the Conservatives, this is the kind of seat that Poilievre will need to win in the next election to form a government. The only time the Conservatives have won this part of Mississauga over the last few decades was in 2011, when Stephen Harper formed his only majority government, but it ranks 149th on the list of ridings ordered by the Conservatives’ margin of victory or defeat in 2021.

That puts it in tipping-point territory for either a Conservative or Liberal plurality.

This is why Mississauga–Lakeshore is so important for the Liberals. The party’s dominance of the Greater Toronto Area is what has kept it in office since 2019. Losing that dominance probably means the Liberals don’t win more seats than the Conservatives do.

Results over the last 20 years have been pretty consistent, with only minor swings between the Liberals and Conservatives. With the exception of the 2011 election when the NDP ate into the Liberal vote, the Liberals have been able to maintain at least 44% support. That’s normally enough to win anywhere.

The Liberals landed a star candidate in Charles Sousa, who won this seat for the Ontario Liberals in 2007, 2011 and 2014 and was finance minister from 2013 until 2018, when Kathleen Wynne’s government (and Sousa, personally) went down to defeat.

Ron Chhinzer, a police officer, is the Conservative candidate, while Julia Kole, an Ontario NDP constituency assistant, is running for the New Democrats. Community advocate Mary Kidnew is the Green candidate and IT professional Khaled Al-Sudani is running for the People’s Party.

Voters in Mississauga–Lakeshore will have to work hard to search these names out as there are also 34 Independents on the ballot, nearly all of them part of a campaign to protest the government’s abandonment of electoral reform.

Trudeau’s byelection performance about average

This test will be the 19th byelection held since Justin Trudeau first came to power in 2015. Over that time, his record has been rather middling — and in the danger-zone for prime ministers.

Lots of very savvy politicos like to say that byelections don’t matter. I couldn’t disagree more.

These are real voters casting real ballots, a political act infinitely more important than polls or political spin. Yes, byelection turnout is lower than in a general election, but there is no reason to believe that the people who vote in byelections are politically all that different from people who vote in general elections. That results between general and byelections are broadly similar suggest exactly the opposite.

And, historically, good byelection performance tends to correlate to good general election performance.

A few years ago, I assessed how Trudeau’s byelection record stacked-up against past prime ministers. I’ve now updated those numbers with the byelections that have taken place since 2018.

On average, the Liberals have performed three percentage points worse in byelections than they did in the previous general election in the same ridings. That nearly matches the average performance of all prime ministers going back to Confederation.

But it also puts Trudeau at the lower end of the scale.

The top performer was Jean Chrétien, whose Liberals gained an average of 2.3 points per byelection throughout his time as prime minister. Robert Borden (2.2 points) and Lester Pearson (2.1 points) also got good results when they were in office. Wilfrid Laurier (0.6 points) was the only other prime minister to average a gain in byelections his party contested.

Everyone else lost support, with John A. Macdonald, Mackenzie King and Alexander Mackenzie seeing their party drop by about two points in every byelection.

On average, the governing party has lost 2.9 percentage points in byelections. Trudeau’s average loss of three points is just below that and ties him with Stephen Harper.

The rest of the list did much more poorly, with Louis St-Laurent, Pierre Trudeau, John Diefenbaker and R.B. Bennett losing between six and eight points. The worst byelection performer — and with a decent sample size of byelections to boot — was Brian Mulroney. On average, his Progressive Conservatives lost more than 25 points in every byelection held during his tenure.

Trudeau’s win-loss record is better

Even though changes in vote share are, in my opinion, the best barometers of what a byelection means, byelections are generally scored in wins and losses. By that measure, Trudeau does better.

Excluding byelections in which the Liberals retained a seat they previously held or did not win a seat they previously did not win, Trudeau is a net +2 since 2015. His party has only lost one seat in a byelection (Chicoutimi–Le Fjord in 2018) but has gained three: Lac-Saint-Jean and South Surrey–White Rock in 2017 and Outremont in 2019.

That +2 rating puts Trudeau tied with Laurier for fourth in the prime ministerial rankings, behind John Abbott (+15), Macdonald (+8) and Harper (+4).

That +/- score isn’t very revealing, though. Some of the winningest prime ministers (St-Laurent, Pierre Trudeau and King) had awful +/- scores, while the top performers measured by vote share include the likes of Chrétien, Borden, Pearson and Laurier, who between them only lost one general election after becoming prime minister.

Losing in byelections is bad

Breaking the results down by terms in office, we see that governing parties that perform well in byelections are more likely to win the next general election than governing parties that perform badly.

That might seem glaringly obvious. But it shows why byelections do matter.

When governing parties have increased their vote share in byelections throughout a term in office, they have been re-elected as the governing party in the subsequent general election 14 times. Only once have they been defeated (King’s Liberals in 1930).

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When governing parties have decreased their vote share in byelections, they have been defeated in the next general election 13 times and re-elected 14 times, three of those victories being a reduction from majority to minority status.

If we take that demotion as a “loss”, then a bad byelection record between general election has meant dropping a rung on the ladder 16 out of 27 times, or about three-fifths of the time.

That’s the silver lining for a government struggling through byelections. It doesn’t mean defeat is imminent, but it does mean it is more likely. As always, it’s much better to be winning.

Watch your inboxes tomorrow morning for my analysis of the results. And don’t forget to tune-in to the livestream tonight!

Trudeau's mixed byelection record to be…