VANCOUVER - Jack Layton says broadening the appeal of the NDP won't involve cutting its traditional ties to the labour movement.

The NDP leader dismissed suggestions Saturday that the tight relationship with labour unions is an impediment to the party's growth.

Indeed, Layton credited the relationship for the historic gains made in the May 2 election, in which the NDP scored a record 103 seats and vaulted into official Opposition status.

"I think we've come to where we are because of those positive ties and working together for working families," he told reporters during the second of a three-day NDP policy convention.

"That's our priority and continues to be. It's been there since our founding and we've now achieved the best success we've ever had electorally. So I think you want to continue with what's working."

The convention is being billed as the first step toward making the leap from opposition to government by the next election in four years.

In a bid to broaden its appeal, New Democrats are debating whether to jettison some of their ideological baggage, including excising the word "socialist" from the party's lexicon.

Delegates are being asked to approve a new preamble to the party's constitution, which touts the "social democratic principles" of economic and social equality, individual freedom and responsibility and democratic rights. That would replace the current preamble's reference to the principles of "democratic socialism," which include "social ownership" and a pox on making a profit.

The proposal, which is to be voted on Sunday, is running into stiff opposition from delegates who believe the party is denying its own roots.

There is no debate, however, on the role of labour unions, which were key players in the founding of the NDP 50 years ago.

Ken Georgetti, president of the Canadian Labour Congress, said the relationship has ensured the NDP has always stood up for "ordinary Canadians."

"It's the reason why our party was founded 50 years ago by the CLC and the CCF," Georgetti said.

"We had a shared vision, a vision of a prosperous and progressive country where everyone shared in the wealth that we all produced, a country with a political party that would stand up for the interests of ordinary working people, a political party that would fight for laws to give them fundamental rights at work."

If anything, labour disputes at Canada Post and Air Canada seem to have strengthened the ties between labour and New Democrats, who routinely refer to one another as "brothers and sisters" at the convention.

Layton himself took a break from the convention Friday to deliver a message of solidarity with striking postal workers.

He blasted Prime Minister Stephen Harper again Saturday for introducing back-to-work legislation last week immediately after Canada Post locked out workers. Until then, postal workers had been conducting rotating strikes to minimize disruption to mail service.

"Why has he closed the door on Canada Post? Here's a guy who says he was a terrific manager of the economy and all things economic yet he's shut down our postal service," Layton told reporters.

"It's certainly the wrong thing to do and it sends a very bad signal out to the working people that tromp up and down our sidewalks and deliver our mail ... that he doesn't really respect the bargaining process."

Layton vowed to use all "parliamentary tools that we have at our disposal" to try to head off the back-to-work legislation. And a top adviser, Brad Lavigne, said staging a filibuster is an option that hasn't been ruled out.

However, with Harper's Tories holding a comfortable majority, Layton acknowledged it will be a "tough row to hoe."

The Harper government also served notice last week that it would legislate Air Canada back to work, of back-to-work legislation last week, less than a day after contract talks broke down. Air Canada reached a tentative settlement on Thursday.

Georgetti said the Harper government's handling of the Canada Post and Air Canada disputes demonstrates "the reasons the New Democratic Party was created haven't changed a bit."

"Workers, union and non-union alike, need a political party in their corner."

The federal NDP also received some advice from Nova Scotia Premier Darrell Dexter, who led his provincial New Democrats to victory in 2009.

Dexter urged the party not to take its historic breakthrough for granted, and to focus on proposing realistic policies that will appeal to a broad segment of voters.

"You must hold to account the government of the day, but you must also demonstrate to Canadians that our party can be entrusted to form government," Dexter said in a speech Saturday afternoon.

"As New Democrats, we must not fall into the trap of believing that we are owed anything. That is an outlook for others. Every vote, every seat, must be earned by the choices we make over the next four years."