OTTAWA, June 30 (Reuters) - Justin Trudeau's Liberals picked up another seat in the Canadian House of Commons on Monday, but more importantly added to their claim to be the main alternative to Prime Minister Stephen Harper's Conservatives ahead of the 2015 general election.
Four vacant House seats were filled in Monday's elections. The Conservatives held on to two in their stronghold of the western province of Alberta and the Liberals held one Toronto-area seat, but seized another Toronto district from their rivals on the left, the New Democratic Party (NDP).
The NDP had leapfrogged the Liberals in the 2011 election to become the official opposition under popular leader Jack Layton. But he died soon afterwards and successor Thomas Mulcair has struggled to match his popularity.
That has proved particularly true as Trudeau, son of charismatic former Prime Minister Pierre Trudeau, surged to the fore after taking over as Liberal leader last year. Under Trudeau, the Liberals lead both the Conservatives and the NDP in national polls.
Both Trudeau and Mulcair campaigned heavily in the Trinity-Spadina electoral district in downtown Toronto that Layton's widow Olivia Chow held until she stepped down to challenge Rob Ford for mayor of Toronto.
The race became a popularity contest for the two leaders, and though Trudeau still has far fewer seats in the House, the result gives him momentum and he will be certain to claim he is the standard-bearer for those who want to replace Harper.
One of the reasons Harper secured a majority of the seats in 2011 was the split of the vote between the NDP and the Liberals. Many Conservatives had secretly been rooting for the NDP to hold its own on Monday in order to keep a lid on Trudeau.
The Liberals were helped in the Trinity-Spadina contest by the fact that their candidate was high-profile city councilor Adam Vaughan, who has won his municipal seat with enormous margins.
The next general election is scheduled for October 2015. (Editing by Ron Popeski)