Justin Trudeau's Liberal Party emerged victorious in Monday's by-election races, stealing a seat from the NDP and boosting its vote share across the board.

Liberal candidate Adam Vaughan, a former city councillor and broadcaster, nabbed the Toronto riding of Trinity-Spadina from the NDP, while Liberal Arnold Chan boosted the party's vote in holding the long-time stronghold of Scarborough-Agincourt, another Toronto riding.

The Conservatives held both their seats but did so while fending off a Liberal surge in Fort McMurray-Athabasca and seeing their vote share decline in all four races.

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Voter turnout Monday was also exceptionally low, ranging from 15 per cent in Fort McMurray-Athabasca to roughly 30 per cent in Scarborough-Agincourt.

A victorious Mr. Vaughan told The Globe and Mail Monday evening that the results are a sign of the Liberals' focus on cities.

"The urban agenda is back, the Liberal Party is on the rise, and what won us the election tonight will win us the elections we need to form government in 2015," Mr. Vaughan said, later adding: "I wouldn't have campaigned as hard, and I wouldn't have won as strong a vote as I did, if Trudeau didn't have one enormous following in this part of the city."

The Liberal Party has boosted its share of the vote in all nine by-elections since Mr. Trudeau became leader last year, winning two new seats – Labrador from the Conservatives last year and Trinity-Spadina from the NDP on Monday.

"We think that's an indication we're headed in the right direction, but there's an enormous amount of work to be done," Gerald Butts, Mr. Trudeau's principal adviser, told The Globe Monday evening.

Of Monday's four by-elections, the Conservatives held Fort McMurray-Athabasca, though candidate David Yurdiga received a far lower share of the vote than his predecessor, and Macleod, where John Barlow won easily.

The party took 72 per cent of the vote in Fort McMurray-Athabasca in 2011 but less than 50 per cent on Monday. Conservative Party spokesman Cory Hann congratulated the new MPs and emphasized that the party held Fort McMurray despite a major push there by the Liberals, who dispatched Mr. Trudeau three times to the riding.

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"Voters instead chose the strong, stable leadership of Prime Minister Stephen Harper and our Conservative Party. These results show that Canadians know Justin Trudeau is in way over his head. Our strengthened caucus will continue to focus on creating quality jobs, lowering taxes and growing the economy," Mr. Hann said in his statement.

In an interview, Mr. Barlow said his resounding victory – he earned two-thirds of the vote – was due to old-fashioned campaigning.

"We wanted to make sure people knew what was going on, that we didn't take Macleod for granted. I really set out from the beginning saying I wanted to earn every single vote," he said, adding his No. 1 priority is rebuilding in a region battered by last year's floods in Alberta.

He declined comment on the Fort McMurray results, but said the Conservatives remain the foremost champion of Alberta's energy sector. "Especially now, it's about having a prime minister that supports the oil sands, supports pipelines and wants to follow through on those projects. Especially for Alberta right now, those aren't wants, they're needs," he said.

The night saw Liberal fortunes rise at the expense of both other major parties. The Conservative share of the vote dropped in all four ridings, as compared to the 2011 election, as did that of the NDP.

"People want a united, strong, progressive alternative to Stephen Harper, and we've been working hard to be that. So we're pleased with tonight, but it's a step along the way," Mr. Butts, the Trudeau advisor, said, adding the party's stronger showing in Fort McMurray will be part of an ongoing push for the Liberals in the West.

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"We're not going to woo people with a grand pitch on one great occasion. We are going to make incremental progress as time goes by and we're going to keep coming," Mr. Butts said.

The NDP had poured resources into Trinity-Spadina, the former riding of party stalwart Olivia Chow, but nonetheless saw candidate Joe Cressy lose by nearly 20 per cent. NDP Leader Thomas Mulcair also visited Scarborough-Agincourt, where the NDP assembled a campaign team with ties to its 2011 breakthrough in the city, but the party had only 8 per cent of the vote there Monday.

Mr. Cressy congratulated Mr. Vaughan on Monday, a sentiment echoed by the party's national director, Anne McGrath.

"I think we had great candidates and ran really strong campaigns, really good campaigns," she said. "[Mr. Cressy] was up against a very well-known city councillor … so it was a tough race."

She noted the party lost a by-election in Winnipeg in 2010 before its breakthrough in 2011's general election.

"Obviously, it's never good when you don't win in by-elections, but we've had that experience before. All parties have had that experience before," she said.

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Mr. Vaughan did not mince words when asked what the results meant for the NDP.

"You saw the same thing happen to the NDP tonight as you saw happen to the NDP provincially [in Ontario]. When you play politics and you build a party instead of building cities and delivering results, you're going to get hurt at the ballot box. What matters is actually delivering results to people, and Liberals know that when they govern from a position of delivering real programs to real people, majority government is theirs. That's where we're headed and that's what we're looking forward to," he said.

The next federal election is scheduled for the fall of 2015.