There's always something to learn from byelections. For starters, Jagmeet Singh will find out whether he has a future as the leader of the NDP.

But the results of all three of today's byelection contests also will shine a spotlight on some of the key dynamics that could decide the 2019 federal election.

Local factors always play a significant role in byelections, making it hazardous to draw overly broad conclusions from the results. But they're still instructive.

We know, for example, that how a party performs in byelections offers clues to how they might fare in subsequent general elections, that prime ministers with better byelection records have a higher chance of staying in office and that these seemingly minor affairs can have enormous historical significance.

There's certainly the potential for some drama tonight.

Singh is staking his leadership on the voters of Burnaby South, a riding in British Columbia's Lower Mainland. The area has voted for the NDP in every election since 2004 and even has a history of electing NDP leaders in need of a seat: Tommy Douglas won here in a 1962 byelection after he failed to secure a seat in his native Saskatchewan. Singh, a former Ontario MPP, is hoping to be offered the same hospitality.

If he doesn't get it, his days as leader of the NDP are likely to be numbered.

Can Singh put a halt to the NDP's struggles?

It speaks to a larger problem for the New Democrats. The party has made little headway since Singh became leader in October 2017. It's been dropping in the polls and now sits at just 14 per cent in the CBC Poll Tracker — a result which, if replicated on election night, would be the worst result for the party in nearly 20 years.

Singh's approval rating is in the red and he stacks up poorly against Prime Minister Justin Trudeau and Conservative Leader Andrew Scheer on the question of who Canadians would prefer in the top job.

This has a wider impact on the electoral landscape. The Conservatives have a tough time winning elections when the NDP is weak.

Expectations are that Singh will win in Burnaby South. His party has poured its meagre resources into the riding and Singh — who has taken up residence in Burnaby — has been campaigning there for months.

There's no telling what impact a defeat there would have on the NDP. It could send the party into a tailspin from which it couldn't recover in time for the fall election. Or the party could install an interim leader with better prospects.

How to follow tonight's byelection results

Join us for results and analysis on a live Power & Politics byelections special, hosted by Vassy Kapelos and Éric Grenier, starting at 10 p.m. ET on cbc.ca, YouTube, Facebook and Twitter. You can also follow results on a special edition of The National with Ian Hanomansing on CBC Television and CBC News Network - and check for updated results stories throughout the evening at cbc.ca/politics

With that kind of uncertainty looming over the party, the New Democrats might be satisfied with a win of any kind. The party needs some good news. If Singh can secure the seat by a comfortable margin, it could be the signal needed to give the NDP hope that the rest of the year will be better.

A squeaker of a win, however, would only mean that a lot of work remains to be done before the NDP starts seeing some light at the end of the tunnel — and the Conservatives start breathing easier.

Battleground GTA - if Bernier doesn't get in the way

The Conservatives are unlikely to lose any sleep over York–Simcoe, a riding located north of Toronto. It has been a safe seat for the party ever since the right was united in 2003. The Conservatives won it by 45 points in 2011 when Stephen Harper secured a Conservative majority government. They still held it by 12 points in 2015 when Harper went down to defeat.

While it's a more sprawling riding than many of those located throughout the Greater Toronto Area, it's still only a stone's throw away from some of the seats that decide every federal election. So while the results might not be emblematic of the battles that will be fought between the Liberals and the Conservatives in the GTA in the fall, they might still give us a clue to just how hamstrung the Conservatives could be in the region by Maxime Bernier.

Andrew Scheer's Conservatives are expected to hold the Ontario riding of York–Simcoe, but it's the results for Maxime Bernier's People's Party that will offer clues to how likely the Conservatives are to make gains in the rest of the Greater Toronto Area. (Darryl Dyck/Canadian Press)

The runner-up in 2017's Conservative leadership race still has a long way to go before his People's Party is in a position to challenge for anything but Bernier's own seat in the Beauce region of Quebec. The Poll Tracker pegs support for the PPC at just 2.5 per cent nationally.

But Bernier could still have a significant impact on the Conservatives' chances of winning seats in key regions of the country — such as the GTA.

A recent Innovative Research Group poll gave the PPC five per cent support in the suburban GTA. With Liberal support slipping in Ontario in the wake of the SNC-Lavalin affair, a lot of their seats in the GTA are up for grabs. But fewer of them will flip over to Scheer's party if Bernier scoops up five per cent of the vote that otherwise would have gone to Conservative candidates.

The results in York–Simcoe will be a good gauge of just how much of a complication the People's Party will prove to be for Scheer in October.

Quebec and the Liberal path to re-election

If the Conservatives are banking on the GTA to propel them back to power (and are hoping Bernier stays out of the way), the Liberals are looking to Quebec to keep themselves in office.

The New Democrats won 16 seats in Quebec in the 2015 federal election. With their support in the province plummeting to just 8.9 per cent, according to the CBC Poll Tracker, the party is on track to lose every single one of them.

The Liberals are counting on that; they need gains in Quebec to offset expected losses in places like New Brunswick and Ontario. With 38.3 per cent support and a 17-percentage point lead over the Conservatives in the province, the Liberals are well-positioned to win most of those 16 seats.

Prime Minister Justin Trudeau is counting on Quebec to help him secure re-election in the fall. That plan starts with Outremont on Monday. (Paul Chiasson/Canadian Press)

Outremont would be high on that list of potential pick-ups. Prior to Tom Mulcair's breakthrough there in 2007, the riding was a Liberal stronghold. It's also the type of urban, diverse riding the Liberals tend to hold throughout the country.

An NDP win would be a tremendous upset and a signal that the Liberal plan for re-election is in big trouble. So Trudeau doesn't simply need to win Outremont — he needs his party to win it with a crushing margin. The provincial polls suggest that the Liberals should win the riding easily. They'll want to see a big number there, and a small one for Singh's NDP.

They'd also like to see middling numbers for the Bloc Québécois, which is enjoying an uptick of its own under new leader Yves-François Blanchet. The Bloc was the perennial rival of the Liberals in Outremont before its collapse in 2011.

So when the results of Monday's three federal byelections start to come in, don't just look at who won and who lost. The implications might go far beyond that. In an election year, every tea leaf tells a story.

WATCH | What's at stake in Monday's byelections?