Liberal Leader Justin Trudeau has emerged as the challenger to Prime Minister Stephen Harper ahead of the 2015 federal election, experts say, after his party held two ridings in Monday’s byelections and improved its fortunes significantly in Conservative strongholds.

Voters cast ballots in four byelections Monday, and the results maintained the status quo in terms of seats: the Liberals held Toronto Centre and the Montreal riding of Bourassa, while the Conservatives retained the two Manitoba ridings of Provencher and Brandon-Souris.

But the Liberals improved their vote share in all four ridings compared to the 2011 federal election, and firmly planted themselves as the alternative to the Conservative government.

“At the end of the day, everybody has what they started with,” CTV’s political commentator -- and communications director for former prime minister Paul Martin -- Scott Reid told Canada AM on Tuesday.

“But we do know that the political firmament is realigned. We know now that Justin Trudeau can win, he is the designated challenger and that’s got to be concerning for the NDP, but it’s also got to be concerning for the Liberals, too. Today is the first day of the rest of your political life, Justin Trudeau.”

Although the Conservatives held Provencher and Brandon-Souris, the Liberals proved themselves to be the second-choice for many voters in the two western ridings.

The most stunning change was in Brandon-Souris, where the Liberals went from winning about 5 per cent of the vote in 2011 to 42 per cent this time around. The winner was declared after only all polls reported, in the early morning hours of Tuesday, with only a few hundred votes separating winner Larry Maguire and Liberal challenger Rolf Dinsdale.

The Conservatives had a more comfortable victory in Provencher. However, Conservative Ted Falk won with 58 per cent of the vote, down 12 percentage points from 2011, while the Liberals garnered about 30 per cent of the vote, up about 23 points from 2011.

The jump in vote share was much less significant in the Liberals’ traditional strongholds of Toronto Centre and Bourassa. But the victories there were significant because the NDP mounted strong challenges in each riding with big-name candidates and clearly had high hopes to steal at least one riding as evidenced by Leader Tom Mulcair’s frequent trips to each city to campaign.

In the end, Chrystia Freeland won with 49 of the vote to win Toronto Centre, while NDP candidate Linda McQuaig garnered about 36 per cent. In Bourassa, Emmanuel Dubourg won with about 48 per cent support compared to NDP candidate Stephane Moraille’s 32 per cent.

The Conservatives captured less than five per cent of the vote in Bourassa and about nine per cent of the vote in Toronto Centre.

“It is the Liberal Party tonight that proved that hope is stronger than fear, that positive politics can and should win out over negative,” Trudeau told supporters in Bourassa late Monday night.

The night’s results are troubling for the Conservatives and the NDP, but for different reasons.

Mount Royal University political science professor Duane Bratt said combined with the Liberals’ strong showing in last year’s Calgary Centre byelection and the fact the party nabbed former Conservative MP Peter Penashue’s old seat in Labrador shows that the Grits are getting “closer and closer to the Conservatives.”

“The trend line at least from the byelection is that we’re going back to a Liberal-versus-Conservative battle, which has been Canada’s history, and the NDP look like they’re going to be falling from their Official Opposition status,” Bratt told CTV News Channel Tuesday.

The Senate scandal appears to be dragging down the Conservatives’ fortunes.

“Whether it’s the Senate scandal, whether it’s the prime minister’s personal falling numbers, there’s something that’s not right in Conservative land and it’s got to concern them,” Reid said.

“The air is out of this balloon and they’ve got to figure out how they use the next two years to reanimate their electoral appeal.”

For Mulcair, “he’s got to be very disappointed” with the results given that his party trailed the Liberals in all four ridings combined by an average of 22 per cent.

Mulcair has a big task ahead of him to prepare for the 2015 election if he wants to protect the gains made by the party in 2011, when the so-called “Orange Crush” swept Canadian politics and catapulted the NDP to official opposition status, largely on a gain of nearly 60 seats in Quebec.

“The ridings that the Liberals have held onto have been traditional Liberal ridings,” Bratt said. “We’ll have to see about these new ridings that were taken away from the Bloc Quebecois, what a Liberal-NDP race can look like and we haven’t seen any byelections that look like that yet.”

Mulcair has emerged as the star of question period with his repeated hammering of the government over the Senate expenses scandal. However, it appears that Trudeau and the Liberals have been the ones to benefit.

“He’s been the guy that’s taken it to the government and he wasn’t able to cash that into seats and pickups and electoral victory yesterday,” Reid said.