NDP president Brian Topp is running to replace Jack Layton as leader, having already secured the support of Quebec MP Françoise Boivin and former leader Ed Broadbent.
The trio appeared together at a news conference in Ottawa ahead of an NDP caucus meeting in Quebec City this week.
Ms. Boivin and Mr. Broadbent's endorsements came as a surprise, but are a clear signal of Mr. Topp's strengths and wide-ranging support in NDP circles.
It also shows that not all of the NDP's Quebec MPs will support the eventual leadership bid of NDP House Leader Thomas Mulcair.
Ms. Boivin was a Liberal MP from 2004 to 2006, who went on to run for the NDP in 2008. She won a seat for the party last May as part of the "orange wave" that swept up most of Quebec.
Mr. Mulcair, who has yet to officially announce a leadership bid, has presented himself as a key factor in the NDP's strategy to keep the party's Quebec seats in the next election.
Mr. Topp was a close ally of Mr. Layton until the former leader passed away last month. He grew up in Quebec, but worked in Saskatchewan through much of the 1990s and is now based in Toronto.
"I know that New Democrats can deliver a successful NDP government in both official languages and in every part of the country," Mr. Topp said. "A fiscally responsible and economically literate government that offers Canadians hope for a better future."
In his opening statement, he offered a tribute to Mr. Layton, calling him a hero and great leader, promising to follow in his footsteps.
In a prepared statement, Mr. Broadbent said of Mr. Topp: "He's a listener and a team player and he knows how to get things done. He will be a great leader of our party."
Ms. Boivin highlighted Mr. Topp's roots in Quebec, which is home to 59 of the party's 103 seats.
"He understand Canada and he understands Quebec. That's key to building our party and winning the next election," she said.