If you're looking for clues on what to expect in next year's federal election, byelections are a good place to start.

Yes, individual byelections can hinge on local issues and well-known candidates, making it perilous to draw broader conclusions from the results. But they're also the only moments between general elections when real voters are casting real ballots — when a party's messaging, strategy and leadership are put to the test.

And when those individual contests are taken together, they can tell a compelling story. For Justin Trudeau's Liberals, it isn't clear whether it's going to be a story with a happy ending in 2019.

The next chapter will be told through Monday's federal byelection in Leeds–Grenville–Thousand Islands and Rideau Lakes. Polls are open from 8:30 a.m. until 8:30 p.m. in the eastern Ontario riding.

An upset is unlikely. Held by Conservative MP Gord Brown from 2004 until his death earlier this year, the riding has voted blue ever since the old Progressive Conservative and Canadian Alliance parties merged in 2003.

The Liberals held the riding in the 1990s, but they were helped by a divided field. The last two times the Liberals won what was then Leeds–Grenville — in 1997 and 2000 — they did so with less than 40 per cent of the vote, while the two right-of-centre parties together pulled a majority of votes cast.

But that doesn't mean there won't be something worth watching in Monday's results. There's always something to learn from how a party's share of the vote has shifted from the previous election.

Justin Trudeau falls near the middle when it comes to changes in vote share in byelections. (CBC News)

This will be the 14th byelection of the current Parliament. From those, the Liberals gained two seats in 2017 (South Surrey–White Rock in B.C. and Lac-Saint-Jean in Quebec) and the Conservatives picked up one earlier this year (Chicoutimi–Le Fjord, also in Quebec). Incumbent parties have held on to the other seats.

On average, Trudeau's Liberals have lost 1.2 percentage points in these byelections compared to the 2015 election results in these ridings. That puts Trudeau somewhere in the middle of the field, historically speaking.

An analysis of over 500 byelections held since 1867 (see the methodological note below) shows that Trudeau's byelection record places him sixth out of 17 prime ministers who contested at least one byelection, and for whom byelection results are available. (The Library of Parliament does not record detailed results for byelections held between 1891 and 1896).

The governing party has lost an average of 2.9 points in byelections, indicating that Trudeau's Liberals are beating the historical average for a sitting government.

But consistently strong byelection performances are relatively rare for governments. Only three prime ministers averaged a gain of more than one percentage point throughout their times in office: Robert Borden, Lester Pearson and Jean Chrétien, who was the top performer with an average 2.3 point gain in byelections.

The worst was Brian Mulroney, who lost an average of 25.5 points in byelections. No other prime minister who contested at least five byelections hit double-digits in average losses.

Trudeau narrowly beating average performance of first-termers

But the sample size for the Trudeau Liberals remains relatively small compared to those for other prime ministers, some of whom contested over 30 byelections during their tenures. And Trudeau is a first-term prime minister, which might put him in a better position than longer-serving prime ministers who have worn out their welcome.

Looking at each prime minister's performance by term again puts Trudeau in the middle. His first term in office ranks 20th out of 41 analyzed terms. Among 16 first-termers, Trudeau ranks eighth.

The governing party led by a prime minister in his first term has averaged a loss of 1.6 points in byelections, suggesting that Trudeau is performing only slightly better than the average first-term prime minister.

The top performers did significantly better. Louis St-Laurent averaged a gain of nearly nine points per byelection in his first years in office. Wilfrid Laurier picked up an average of just under seven points in byelections held between 1896 and 1900.

And in their second terms, Stephen Harper gained an average of 6.3 points between 2008 and 2011, while Chrétien increased the Liberals' vote share by 8.3 points in byelections held between 1997 and 2000.

Mulroney again lands at the bottom of the pack, having lost an average of 24 points in byelections held during his first term and 27 points in his second. But other prime ministers also struggled mightily. William Lyon Mackenzie King's Liberals dropped 15 points in byelections held during the Second World War before winning re-election in 1945.

But in other cases, byelection difficulties presaged looming defeats. John Diefenbaker's PCs lost an average of 10 points in byelections between 1958 and the general election in 1962 — when the party was reduced from a landslide majority to a slim minority. Harper's Conservatives lost an average of 11.5 points in byelections held during his last term in office.

Byelection winners have more general election success

In fact, there does seem to be a relationship between byelection performance and how a government does in a subsequent general election.

This is where things get complicated for Trudeau. Of 15 prime ministers who saw an average increase in their share of the vote in byelections during a single term, only one of them went on to defeat. That was King in 1930, when the country was in the grips of the Great Depression.

Of the 25 who lost vote share in byelections, however, the result is split down the middle — 12 were re-elected and 13 were defeated. This is where Trudeau finds himself 11 months out from next year's federal election.

But his win-loss record puts him in a better position. Of the 12 who ended a term batting above .500, only one of them went on to be defeated — King again — while John Abbott, who was prime minister from 1891 to 1892 and put up a very winning record, was out of office (and replaced in succession by John Thompson, Mackenzie Bowell and Charles Tupper) before his party went on to defeat in 1896.

Of the 21 prime ministers who lost seats during their terms, only five saw their majority governments re-elected; four were reduced to a minority and 12 were defeated.

Conservative Leader Andrew Scheer is hoping to hold the riding of Leeds–Grenville–Thousand Islands and Rideau Lakes in Monday's byelection to replace the late Conservative MP Gord Brown. (CBC News)

This is why the results of the Leeds–Grenville–Thousand Islands and Rideau Lakes byelection on Monday — and the remaining byelections the government plans to hold in February — could help clarify Trudeau's historical odds of re-election in 2019.

He sits in the murky middle when it comes to how his party has fared in the popular vote. Those above him in the rankings were nearly all re-elected. Those at the bottom of the rankings were nearly all defeated. Trudeau's spot puts him in mixed company — not much better than Harper before his re-election in 2008 or Laurier before his defeat in 1911, and about even with King when his party lost the 1925 election (but held on to power in a minority legislature).

It's perhaps an unsatisfying result if we're hoping for these byelections to give us clues about who will win in 2019.

But the lack of a definitive conclusion is a clue itself — that next year's election is defying easy predictions.

All byelections held since Confederation have been included in this analysis if the governing party ran candidates in both the preceding general election and the byelection vote, and if the seat was not won by acclamation in either election. All cases in which there were multiple candidates from the governing party, or a lack of clarity on party affiliation, were excluded.