The 39-year-old MP from northern British Columbia billed himself as an anti-establishment contender who will put an end to Prime Minister Stephen Harper's brand of aggressively partisan politics.
"Make no mistake. I don't just want to beat Stephen Harper. I want to beat the way he does politics," Cullen said Friday, slamming what he described as the prime minister's divisive, "wedge" approach to politics.
"We don't need a prime minister who is his own strategist. Politics is more than that."
But Cullen's pitch was also aimed at denting the lead of perceived frontrunner Brian Topp, a longtime backroom strategist and key architect of the late Jack Layton's electoral successes.
Topp has racked up endorsements from an impressive roster of NDP luminaries, including former leader Ed Broadbent and former Saskatchewan premier Roy Romanow.
Just hours before Cullen announced his leadership bid in Vancouver, Topp was on Parliament Hill to introduce his latest backer — veteran Vancouver MP and deputy NDP leader Libby Davies.
Davies, who had mulled making a leadership bid of her own, said her choice was not a negative commentary on other declared or potential contenders, including fellow British Columbians Cullen and Peter Julian or fellow deputy leader Thomas Mulcair.
"I feel very confident that Brian Topp has the leadership abilities and the experience and his connections and experience across the country to move us forward to take on Stephen Harper," she said.
In remarks posted on his new leadership website, Cullen took a shot at Topp and his list of influential supporters.
"I know the Ottawa establishment thinks this race is already done," he said.
"The Ottawa establishment couldn't imagine Liberals might lose government. Or that Quebecers would vote federalist again. It's been wrong before. It will be wrong again."
Topp was first out of the leadership gate after Layton died from cancer last month. He was followed by Quebec MP Romeo Saganash. A host of others are expected to join the contest soon.
Ottawa MP Paul Dewar is poised to take the plunge on Sunday, as is a little-known Nova Scotia businessman, Martin Singh.
Julian, who has been testing the waters, has said he'll announce his decision next week.
Mulcair is also expected to eventually enter the race, despite concerns the party's membership numbers are stacked against him. Mulcair's home province, Quebec, has only a tiny fraction of the party's roughly 87,000 members, even though it accounts for more than half the party's 102 seats in the House of Commons.
Toronto MP Peggy Nash, Halifax MP Robert Chisholm and northern Manitoba MP Niki Ashton are also weighing their chances.
The NDP leadership convention will be held March 24 in Toronto.