Paul Dewar officially joined the race to replace Jack Layton as leader of the federal Opposition on Sunday, telling a crowded Ottawa conference room that he would "make a difference."

"Today I am announcing my candidacy for the leader of the New Democratic Party," Dewar told a cheering crowd at Ottawa's Lord Elgin Hotel on Sunday.

"I got involved in politics to make a difference in my community and my country."

Dewar, who represents the Ontario riding of Ottawa Centre, became the fourth person to officially seeking the role of NDP leader.

Under the party's recently-established leadership campaign rules, Dewar will have to relinquish his current assignment as the NDP's foreign affairs critic.

Dewar, worked as a foreign aid worker and teacher before first being elected to the House of Commons in 2006.

He said his focus as party leader, and leader of the Official Opposition, would be to build a stronger economic future for Canadian families and to put the country at the forefront of the climate change battle.

Dewar also acknowledged his limited French is a handicap in a party hoping to build on last May's historic electoral breakthrough in Quebec.

"French is very important to me and I'm working on it every day," he said. "It's a challenge but I will overcome this challenge and take on Stephen Harper in both official languages."

The 48-year-old MP also said that it was finally time the Canadian government achieved true reconciliation and respect with the First Nations, and strengthen health care for all Canadians.

"We share big hopes for Canada. And those big hopes are what we are going to focus on in this campaign," Dewar said on Sunday.

"Jack (Layton) never aimed low, nor must we. Our mission now is to election a New Democrat government and to build a better Canada."

Since the death of Jack Layton in August, just months after he led the party to 102 seats in the House of Commons and its first-ever status as Official Opposition, several high-profile party members have been rumoured to be considering a run for the party's top job. So far, only three others have officially entered.

Brian Topp, a longtime backroom strategist who worked closely with Layton on the recent election is considered the front-runner to win the position.

British Columbia MP Nathan Cullen and Quebec MP Romeo Saganash have also officially joined the leadership race.

Cullen has billed himself as an anti-establishment candidate who will put an end to Prime Minister Stephen Harper's brand of aggressively partisan politics.

Saganash is the first Cree to obtain a law degree in Quebec and a rookie MP who had been named the party's natural resource critic.

MP Olivia Chow, Layton's widow, has confirmed that she will not run for the position and will not endorse a candidate.