New Democrat MP Paul Dewar officially launched his bid for the federal NDP leadership on Sunday, vowing to continue the late Jack Layton's legacy of "building a better Canada."

Dewar, who represents the Ontario riding of Ottawa Centre, announced his candidacy to a loud-cheering crowd of supporters at a news conference at Ottawa's Lord Elgin hotel, joining an increasingly crowded field vying to succeed Layton.

"We're here because we actually care about each other, and we have big hopes for Canada, and those big hopes are what we are going to talk about in this campaign," he said.

"Jack never aimed low, nor must we."

The 48-year-old former teacher becomes the fourth official candidate to join the race, following former party president Brian Topp, Quebec MP Romeo Saganash and B.C.'s Nathan Cullen.

Dewar, first elected to the House of Commons in 2006, cited his previous experience as an educator, union leader, aid worker and grassroots politician.

"These are my roots — helping people to build a better life for themselves and their children."

He also began his speech with a tribute to Nancy Riche, a major figure in the Canadian labour movement and one of the key figures in Newfoundland and Labrador's NDP. Riche died Saturday in hospital in St. John's, of complications from a heart condition.

Crowded field grows

Dewar, who spoke several times in French during his address Sunday, has reportedly been working on his proficiency in recent weeks leading up to the bid to lead the NDP, which made historic gains in May's federal election and now represents the majority of ridings in Quebec.

"What we hear from a lot of our French colleagues is that he still needs to work on that a bit," the CBC's Hannah Thibedeau said Sunday from Ottawa. "Because they do want a leader who is proficient in both official languages."

In order to run, Dewar also has to give up his senior role as the party's foreign affairs critic.

Topp was the first to declare his candidacy to lead the Official Opposition after Layton died of cancer in August and has received several endorsements from high-profile New Democrats, including longtime former leader Ed Broadbent and deputy leader Libby Davies.

MPs Niki Ashton, Robert Chisholm, Peter Julian, Pat Martin, Thomas Mulcair and Peggy Nash are still deciding whether they'll throw their hats into the ring.