Open this photo in gallery Andrea Horwath, leader of the Ontario NDP, joins hands with her son as she speaks after provincial election voting closed, in Hamilton, on Thursday. CHRIS HELGREN/Reuters

From the outside, it appeared like a stinging defeat. Until earlier this week, some polls had the NDP essentially tied with the Progressive Conservatives. But long-time organizers for Ontario’s New Democrats saw Thursday’s results as a huge gain for a party accustomed to coming in third place.

At the Hamilton Convention Centre, where volunteers gathered for NDP Leader Andrea Horwath’s campaign party Thursday, a hush fell over the room just after 9 p.m, after television broadcasts declared a majority for Doug Ford and the Progressive Conservative Party. Still, the crowd perked up as results showed big NDP gains, particularly in Toronto and Windsor.

“This is a historic moment,” said Kathleen Monk, a senior adviser on Ms. Horwath’s campaign, adding that this is one of the best results the party has had in the province.

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“This is not a replay of 2015,” she said. That was the federal election in which then-NDP leader Tom Mulcair launched his campaign with what looked like a clear shot of becoming prime minister. By election night, support had collapsed and the party came in third place. To NDP members, it’s still known as the night that sparked “an existential crisis,” Ms. Monk said.

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This time around, Ms. Horwath and the Ontario NDP launched their campaign as clear underdogs, holding just 18 seats in the legislature. By Thursday night, they appeared poised to at least double their seat count – likely the most won by the party since the only time it formed government, under Bob Rae in 1990. The election results also hand the NDP the role of Official Opposition.

On a practical level, this will translate into heightened influence at Queen’s Park. The role of Official Opposition gives the NDP a much bigger platform in the legislature – seats on committees, and in some cases, the ability to chair committees. “There’s more profile,” said David Wills, a senior vice-president at public relations firm Media Profile who was not involved with the Horwath campaign but has worked on past NDP campaigns. “They do have more of a leadership role, and they set the tone and pace for holding the government to account.”

Open this photo in gallery Andrea Horwath, leader of the Ontario New Democratic Party (NDP), speaks after provincial election voting closed, in Hamilton, Ontario, Canada June 7, 2018. REUTERS/Chris Helgren CHRIS HELGREN/Reuters

Changes made by the Ontario Liberal government last year also mean political parties now receive subsidies based on the amount of support they receive.

In the longer term, NDP members hope Thursday’s results – and the collapse in Liberal support – will represent a fundamental shift in the political landscape.

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“The results position the NDP as the alternative to the PC party,” said Brad Lavigne, a senior adviser to Ms. Horwath’s campaign and long-time NDP strategist. “It’s realigning Ontario politics in a significant way.”

This particular point is especially important for the NDP, said Henry Jacek, a McMaster University political scientist. NDP members have long viewed the path back to government as a two-step process, he said. “First, you go from third-party to Official Opposition. Then the second election, you go from Official Opposition to government.”

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As for whether Ms. Horwath – who has already led the NDP through three general elections – will stay on as leader long term, most supporters seemed to think it’s likely.

“She is the most celebrated leader in Ontario’s New Democratic Party history,” said Ms. Monk. “I think the party has never been as unified behind a leader.”

Matthew Green, a Hamilton city councillor who worked on the campaign and took in the results from the convention centre Thursday, echoed this.

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“Whatever happens tonight,” he said, “Andrea walks away a winner.”

Mr. Jacek agreed that Ms. Horwath likely wouldn’t be replaced anytime soon. He said that aside from a few prominent caucus members – long-time MPPs such as Gilles Bisson, Peter Tabuns and France Gélinas – the NDP doesn’t have many stars who would be obvious contenders as new leaders.

Plus, he added, the NDP hasn’t traditionally rushed to replace leaders, partly because members understand that the public views them as the third-place party.

“The NDP – they know their leader has a hard road ahead,” he said.