NDP MP Nathan Cullen is running for the party's leadership, he confirmed Friday, kicking off a weekend with three expected candidacy declarations.

Cullen is the third official candidate, following former party president Brian Topp and Quebec MP Romeo Saganash. It's expected Ottawa MP Paul Dewar will announce his candidacy on Sunday, with an announcement scheduled for that afternoon a few blocks from Parliament Hill.

A Nova Scotia pharmacist is also expected to declare his candidacy this weekend. Martin Singh is the president of the federal NDP's faith and social justice commission, and the riding president for Sackville-Eastern Shore.

Speaking to Rosemary Barton on CBC's Power & Politics, Cullen said he's comfortable being an underdog, joining the race after Topp has already attracted the support of high-profile NDP members like former leader Ed Broadbent and former Saskatchewan premier Roy Romanow.

"My natural orientation is to building things together with people on a very, very broad spectrum," he said.

"What you'll see is something pretty unique, and that our growth will be pretty exponential toward the end."

Cullen said while he doesn't have cross-Canada name recognition, he doesn't think Topp or Saganash do either. He's embarking on a national tour so people can get to know him. 

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.

Cullen represents the northern B.C. riding of Skeena-Bulkley Valley. He was first elected in 2004 and has served as the NDP's natural resources critic, where he was a vocal critic of a plan by Enbridge to extend its Northern Gateway pipeline through his riding.

He also broke ranks with his party last year to vote in favour of a bill to end the long-gun registry.

The 39-year-old MP is married with twin sons.

On Friday, B.C. NDP MP Libby Davies threw her support to Brian Topp, ahead of her provincial caucus colleague's announcement.