Canada's NDP elects new leader, urges unity TORONTO (Reuters) - Canada's main opposition party chose heavily favored candidate Thomas Mulcair to replace the late Jack Layton, who died of cancer just three months after leading the leftist party to its strongest ever performance in a federal election. Mulcair, a former Quebec cabinet minister who is a relative newcomer to the New Democratic Party (NDP), took over the reins after four rounds of voting at the leadership convention, held in downtown Toronto on Friday and Saturday. The Montreal lawmaker, who advocated the party reach beyond its traditional base to appeal to centrist voters, led all four votes of the day, eventually trouncing runner-up Brian Topp, who campaigned to keep the party squarely left-of-center. "Today we have the greatest human capacity, and the greatest potential to put that capacity to work than at any time in our history," said Mulcair to the crowd of cheering delegates. Our future is limitless, if we get our priorities right. The challenge that confronts us is not a failure of ability or talent, it is a failure of leadership and that is failure that we intend to reverse. Mulcair will now take on the tough job of building on gains made by Layton and proving to Canadians that the traditionally third-place party can defeat Prime Minister Stephen Harper's right-of-center Conservatives in the 2015 election. That's a tall order, partly because the unprecedented surge in NDP votes in the 2011 election was based largely on the drawing power of Layton, sometimes dubbed Smiling Jack. With rifts caused by the sometimes divisive leadership race threatening to destabilize the party, the official message at the weekend convention was one of unity and driving forward to form the Canada's next government. "They told us, before the last election, that it couldn't be done, but we rolled up our sleeves and proved them wrong," said Nycole Turmel in her final speech as the party's interim leader. At every step we have proven them wrong. And today we are just as united and determined as ever. Dislodging the Conservatives may not require actually winning more seats in the House of Commons, but rather holding them to less than half and then trying to form a coalition with the other minority parties. Currently the Conservatives have 165 of 308 seats, or 11 more than half, and the NDP has 102. HOLDING ON IN QUEBEC But with just over 62,000 members casting ballots in the third round from nearly 130,000 eligible voters across the country, the low turnout had Canadian media speculating that the NDP could be losing support. This is a particular risk in the French-speaking province of Quebec, where voters are notoriously fickle and have already started deserting the party and returning to the separatist Bloc Quebecois in public opinion polls. "What the leadership absolutely needs is someone who can keep the momentum that the NDP gained in Quebec, someone that Quebecois voters can connect to on a personal and political level," said Kris Nelson, a new party member from Quebec. Mulcair, one of 10 children born to an Irish-Canadian father and French-Canadian mother, was a provincial environment minister for the Liberal party and in 2007 became the NDP's second-ever member of Parliament from Quebec. The party also faces a tough challenge competing with others on the left, including the Liberal Party, the Green Party and, in Quebec, the Bloc Quebecois. This has led to calls by some in the NDP and Liberals to merge, or at least cooperate in the next election, an idea Mulcair has rejected. Voting was delayed on Saturday afternoon, as suspected hackers attacked the computer system that allowed party members to cast their ballots online. "It was a question of overwhelming our system to make it harder for people to access the site to vote," party president Rebecca Blaikie told reporters, adding: "The auditors have no concern about the security of the vote." Blaikie said all members had the opportunity to cast their ballots and an investigation over the attack was underway. The voting period was repeatedly extended throughout the day as the online ballot system struggled to keep up with users. All counted, in the final ballot Mulcair walked away with 33,881 votes, or 57.2 percent, compared with Topp's 42.8 percent of the vote. In Canada's 2011 federal elections, the New Democrats smashed all expectations, almost trebling its seats in the House of Commons and becoming the main opposition party for the first time ever. The governing Conservatives - who favor low taxes and a tough-on-crime approach - won a majority of seats in the House of Commons in the May 2 election and will not face reelection until October 2015. Additional reporting by Randall Palmer in Ottawa; editing by Todd Eastham