OTTAWA - Tom Mulcair took his fight against the Conservatives straight into Finance Minister Joe Oliver's back yard Friday, hoping perhaps to add a bit of spit and polish to the NDP's less-than-stellar credentials as guardians of the public purse.

Stephen Harper returned the favour, sort of, by travelling to the Northwest Territories - a seat the NDP has held since 2006 - armed with his reputed soft spot for the North and a promise to pave a key highway.

Mulcair said Andrew Thomson would carry the orange banner to challenge Oliver in the riding of Eglinton-Lawrence, saying he'd take Thomson's record over that of Harper's government any day.

"The NDP's Andrew Thomson offers the people of Eglinton-Lawrence something that Joe Oliver hasn't - a record of balanced budgets," Mulcair said as he introduced the party's newest candidate.

As finance minister in Saskatchewan, Thomson brought in big tax cuts in 2006 and engaged in a skirmish with the federal government over equalization payments.

Thomson's candidacy is a clear effort to beef up the NDP's credibility on the economic front - as is Mulcair's other promise: to strengthen the office of the parliamentary budget officer.

"We'll remove the prime minister's power to fire the parliamentary budget officer and make them an independent officer of Parliament, like the auditor general," Mulcair said.

"We'll make transparency the law so that future governments can't hide financial information from Canadians."

Harper, who said a re-elected Conservative government would spend $14 million paving 68 kilometres of the road between Hay River and Fort Smith, N.W.T., also left the door open to Canada participating in a ballistic missile defence program at some point.

"Our position is that we keep evaluating our options," Harper said.

"If we felt that at any point in time that we faced particular threats that required us to participate, that is something we would look at," Harper said. At the present time we haven't made that assessment."

Harper has long made the North a key component of his nine years in office, often using it as backdrops for photo ops and announcements that have a military theme. He's especially fond of using the North to frame his tough talk about Russia and its leader, Vladimir Putin.

Harper argued his government has made investments in Arctic sovereignty, such as military bases and a naval dock, "to make sure we can actually project that sovereignty."

"We are making those investments precisely because those and other threats have existed and we have understood for some time now, long before the invasion of Ukraine, that Mr. Putin's Russia was on a very aggressive course in the world."

Liberal Leader Justin Trudeau wasn't actively out and campaigning Friday. Neither were Mike Duffy or Nigel Wright - but their impact on the race was unmistakable.

Emails released at Duffy's trial show that several senior people inside the Prime Minister's Office were explicitly made aware of Wright's plan to use $90,000 to repay disallowed expense claims on behalf of a recalcitrant Duffy.

That directly contradicts the message Harper has been sticking to for years - that Wright and Duffy were the only ones in the PMO who were in the loop.

"Mr. Harper would have us believe that these were rogue elements within his office who did this without letting him know anything about what was going on," Mulcair told a news conference in Toronto.

"But the thing is, if they were out of control and not reliable people ... how come he's kept them all in his office? That's just not credible."

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Mulcair also lashed out at Harper for gutting environmental protections and easing the approval process for projects like the Energy East pipeline, which he said ended up making it harder, not easier, to get them approved.

Green party Leader Elizabeth May travelled to Nanaimo on Vancouver Island, where she promised a better deal for veterans.

She said she wants to reverse changes to veterans' benefits and said any vet with post-traumatic stress disorder who wants a service dog should get one.