The Liberals won the federal byelection in Lac-Saint-Jean on Monday, wrestling away a Conservative riding in dramatic fashion in a province that will play a key role in Justin Trudeau's 2019 re-election strategy.

And though his party did hold its seat in the Alberta riding of Sturgeon River–Parkland — even increasing its vote share significantly — the results are likely to be a disappointing showing for Andrew Scheer in his first byelection test as Conservative leader.

The Liberals' Richard Hébert prevailed in Lac-Saint-Jean, winning 38.6 per cent of the vote in the riding previously held by the Conservatives' Denis Lebel. That party's candidate, Rémy Leclerc, finished second with 25 per cent, just edging out the Bloc Québécois's Marc Maltais at 23.4 per cent.

Gisèle Dallaire, representing the New Democrats in an early test of Jagmeet Singh's nascent leadership, captured 11.7 per cent of ballots cast. Turnout in the riding was 41.1 per cent.

In Sturgeon River–Parkland, a riding just northwest of Edmonton, the Conservatives' Dane Lloyd won in a landslide, taking 77.4 per cent of the vote to 12 per cent for the Liberals' Brian Gold and 7.7 per cent for the NDP's Shawna Gawreluck.

Turnout in former Conservative interim leader Rona Ambrose's old riding of Sturgeon River–Parkland was just 23.7 per cent.

Liberals' breakthrough in Lac-Saint-Jean

With a gain of 20.2 points over the Liberals' performance in the riding in 2015, Hébert more than doubled his party's support. With the exception of a 25-point gain in the Fort McMurray–Athabasca byelection of 2014, that is the biggest byelection swing towards his party that Trudeau has experienced as leader.

No surprise in Alberta vote, but Quebec riding goes red after nearly four decades 1:09

Much of the Liberals' gain can be chalked up to Hébert, the outgoing mayor of Dolbeau-Mistassini. When Lebel first ran for the Conservatives in a 2007 byelection, the former mayor of Roberval boosted the Tories by 23 points.

But Dolbeau-Mistassini is half the size of Alma, which was expected to be a bastion of support for the NDP and Bloc Québécois. The Liberals in Lac-Saint-Jean were the only party to gain raw votes anywhere on Monday, increasing their total by 3,249 in a riding where turnout dropped by over 20,000. Hébert's profile can explain some of the Liberals' increase, but not all of it.

The party's performance in Sturgeon River–Parkland was much worse, with its share of the vote dropping 3.6 points from 2015. This is in line with the slippage in support the Liberals experienced in a series of byelections in April.

But this is the kind of support the Liberals can afford to lose.

Winning in rural, francophone Quebec — Lac-Saint-Jean was the Liberals' worst riding in Quebec in 2015, it is now their 29th best out of 78 — sends a signal that the party can make up for losses elsewhere in 2019's federal election. And even if a good portion of this win can be chalked up to Hébert, a win like this will make it easier for the party to find more Héberts in the future.

Disappointment for Andrew Scheer

Few deny that Lebel was a star candidate for the Conservatives in Lac-Saint-Jean and his loss was going to cost the party a good chunk of its vote. But a loss is a loss, and Scheer now joins a short list of leaders who have dropped a seat in their first byelection test — a list that includes Stéphane Dion and Robert Manion (a forgettable one-election Conservative leader Scheer would not want to emulate).

Leclerc took 8.3 points less than Lebel did in 2015, the Conservatives' worst byelection performance so far in this parliamentary session and their lowest share of the vote in Lac-Saint-Jean since 2004.

That the Conservatives captured 37.2 per cent of the vote here in 2006 — before the arrival of Lebel — shows how this drop in support cannot all be laid at the feet of the former MP.

The party can be prouder of its win in Sturgeon River–Parkland, where its vote share increased by 7.2 points. But the Conservatives will not win the next general election by increasing its vote share in ridings it already holds.

CBC Polls Analyst Eric Grenier breaks down the results 4:43

Not enough for the Bloc Québécois

The Bloc's third-place finish will be a disappointment for party leader Martine Ouellet, despite the five-point increase in its vote share. That gain is broadly in line with the recent bump in the polls the party has experienced in Quebec.

But the result in Lac-Saint-Jean was still worse than anything the Bloc put up when it won the riding between 1993 and 2006. In Maltais, a local union leader, the Bloc had a candidate with a good profile in a riding in which it has a long history. Even under these ideal circumstances, the Bloc struggled to make a breakthrough.

NDP support slumps in Quebec

Singh's first outing as federal NDP leader was also a struggle.

The New Democrats shed vote share in both ridings, which they have now done in seven of eight byelections since 2015. The slide of 2.3 points in Sturgeon River–Parkland was modest, but the 16.8-point collapse in Lac-Saint-Jean was far greater than the decrease in support the NDP has experienced in the polls provincewide in Quebec.

This should be of great concern to the NDP. The party has 16 MPs from the province, forming the largest cohort in its caucus, and it is key to any hopes the NDP has of repeating its success of 2011.

It is difficult to fault Singh for all of this poor showing, however. Before his arrival, the NDP was already 10 points down in the polls in Quebec from its 2015 result. It is unlikely that any of the other candidates for the NDP's top job would have done markedly better.

Nevertheless, the result suggests that Singh is not in the midst of any early honeymoon — least of all in Quebec, where the NDP needs the help the most.