"I don't think it was serious that after five years of negotiations that was the one day where he had to go to Brussels especially when we know there's two years left to go before the deal is finally concluded," Mulcair said in an interview with CBC Radio's The House.
Harper's departure from Europe comes after the government's speech from the throne.
Mulcair hinted that his suspicions ran deeper.
"Did you think the Europeans didn't know that that might be interesting to him and what did they get in return for that?" he said.
CBC's Evan Solomon dug deeper: "So you think there might have been a trade concession for timing?" he asked.
Mulcair responded, "I don't trust Stephen Harper on trade issues at all. I think the he's always willing to look at his own interest above anyone else's in anything he does especially on a trade issue like this."
"Progress on a trade agreement with Europe is a good thing," Mulcair said, but "the devil's in the details".
'Historic win for Canada'
In Brussels on Friday, the prime minister hailed the accord as an "historic win for Canada."
"Canadian families will have greater access to European goods at a lower cost as 98 per cent of tariffs both ways will be removed immediately" he said.
But the announcement came with a caveat: There's more work to be done.
Speaking to CBC Radio's The House from Brussels, International Trade Minister Ed Fast said, "All of the substantive issues have now been resolved. All that's left to be done, there are a number of technical issues left to be resolved between negotiators. Of course there's also a legal scrub that has to take place."
That will take time, likely "two years," he said. "We're looking forward to getting this agreement ratified and brought into force. The provinces and territories have all confirmed their political support for this agreement."
One provision extending brand-name prescription drug patents could mean the provinces will pay more.
The federal government is promising compensation, but at what cost, isn't clear.
"It's hard to estimate that at this point in time," Fast said. "Obviously as time goes by it will become clear and we'll be working very closely with provinces to make sure they're not out of pocket."
Dairy farmers, worried about more imported European cheese cutting into their business, may get help too.
"If they see a loss of production as a result of this additional market access we will actually compensate them for that," Fast said.
Billions added to economy
What could those potential losses add up to? Mulcair says that's another question for the prime minister.
"That's the type of things we're going to be looking at. We're not going to give him a blank cheque" he said.
The government is touting figures from a joint study that tallied the financial benefits of the bilateral agreement.
"We believe at the very minimum we'll see an additional $12 billion of GDP added to our economy. We expect at least a 20 per cent increase in bilateral trade. It's the equivalent of 80-thousand new jobs. We're confident those estimates are actually conservative, Fast said.
Does the NDP trust those numbers? "Completely theoretical," Mulcair said.
But Fast has faith in them. "We believe that history will bear this out, that this agreement will be a key driver of economic growth and prosperity for Canada," he said.
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