I know better than to read too much into byelection results. For lots of reasons, they are not representative of what a general election in that riding might look like: turnout is lower, parties can campaign intensely when not committed to a national campaign, and it’s a safe way for voters to signal displeasure with a governing party without having to fully embrace an opposition party as worthy of forming government.

So yes, I won’t read too deeply into this week’s byelection results. But I still find the South Surrey-White Rock vote interesting. And if I were a Conservative strategist, I might be a little worried. Not panicked yet, but … wary.

The short description of what happened in B.C. on Monday is that a seat that had been held for a very long time by the Conservatives went to the Liberals. On the face of it, that’s a pretty simple story — Liberal beats Tory. But it misses the reality of what happened.

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The Conservative candidate, Kerry-Lynne Findlay, did about as well as the last Tory to contest the riding, Dianne Watts, who won it in 2015 with 44 per cent of the vote. Findlay was a bit softer than that, but not by much: she took 42 per cent. A drop is a drop, but this wasn’t a catastrophe.

What seemed to make the difference on Monday night was how the NDP did. In a word, badly. In 2015, the NDP came in third in the riding, with 10.5 per cent of the vote.

On Monday, the NDP still came in third. But this time, the party’s share of the vote was only 4.9 per cent. That 5.5 (ish) per cent of the electorate shifted to the Liberal candidate, Gordie Hogg, and that was enough to take the Liberals’ second-place finish in 2015 and turn it into a victory (the Liberals also seemed to pick up some, but not all, of the lost Conservative vote).

WATCH: Federal By-Election: Gordie Hogg wins South Surrey-White Rock

2:58 Federal By-Election: Gordie Hogg wins South Surrey-White Rock Federal By-Election: Gordie Hogg wins South Surrey-White Rock

This is, to put it mildly, a very general analysis. I’m just using raw aggregate numbers here, and I don’t claim any particular knowledge of the riding and the local issues.

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Also, again — I want to stress this point — byelections are not general elections. Turnout was way down compared to the 2015 general vote. So consider those disclaimers made.

Even so, the top-level numbers make for interesting reading.

The Conservatives basically did as well as they did in the last general election in terms of vote share. The Liberals won because the NDP tanked. (No disrespect to Mr. Hogg or his campaign intended — I’m just speaking in terms of the math.)

This is interesting because, after the 2015 election, I noticed something similar. I wasn’t shocked that the Liberals had won. The polls and my own personal gut feeling had already told me that was going to happen. But I was surprised by how strong a win it was.

READ MORE: Trudeau basks in byelection successes while Tories find reasons for consolation

I assumed, at first, it was because of what I felt was a weak effort by a tired and drifting Conservative party in need of a restorative spell in opposition. But once the numbers came out, I was proven wrong: the Conservative vote only dropped a bit in 2015, compared to their majority win four years earlier. They lost about 200,000 votes, but that’s not why Justin Trudeau and his team won the day.

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Two things put the Liberals over the top in such dramatic fashion: a massive number of Liberal voters who either were new voters (new Canadians, those who’d turned 18, etc.) who came out in a big way for Trudeau in 2015, or Liberal voters who’d stayed home in recent elections but decided they liked this young Trudeau; and a significant drop in the NDP vote.

The Conservatives had a crappy night in 2015, with their loss of 200,000, but the NDP got absolutely shellacked. They lost a million votes. The Liberals, it stands to reason, picked a lot of those up.

And that’s a problem for the Conservatives. A strong NDP on the left squeezes the Liberals into the centre, giving the Tories a chance to take the right and enough moderates to win. If the Liberals can take the centre and push it left as far as they want while the NDP retreats, the Liberals win. Every time.

That happened in 2015. That pattern held true again on Monday night in B.C., and shows no sign of reversing, at least not yet.

Two years before the next general, this is what I’ll be watching for: if the NDP, under Jagmeet Singh, can challenge the Liberals on the left, the Conservatives will have a shot. But if the NDP continues to struggle against a Liberal party that has deliberately moved to the left, onto the traditional territory of the NDP, it won’t only be the NDP that suffers. It’ll be the Conservatives.

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The Tories need a strong NDP to win. Right now, they don’t have it.

Matt Gurney is host of Global News Radio 640 Toronto and a columnist for Global News