In the midst of offering his assurances about carbon pricing, Prime Minister Justin Trudeau slipped, and the T-word passed his lips.

"What we are also guaranteeing," he said, midway through his second response in the House on Wednesday, "is that this tax—"

At that, the Conservatives cheered and leapt up to applaud. The prime minister stood, smiling slightly, and waited for the roar to dull.

The acknowledged joke here is that the prime minister insists on referring to a "price" on carbon, even when he is most likely talking about the sort of thing that would be delivered as a "tax" on carbon.

(There is another joke here about how the Conservatives used to support putting a "price" on carbon even while they opposed a "tax" on carbon, but that is a longer story.)

Once the Speaker had restored order, the prime minister corrected himself.

Prime Minister Justin Trudeau trips on his own words by calling his imposed carbon-pricing plan a "tax". 1:14

"Mr. Speaker," Trudeau said, "we have guaranteed that this price on carbon will be revenue neutral for the federal government, and it will be up to the provinces to determine how they choose to reimburse their citizens for this progress on the environment and the economy."

But whatever the wordplay, climate change is a morality play.

Leadership vs. grocery bills

On Tuesday, the prime minister had emphasized the word "leadership."

As in, "Canadians in every province expect leadership from all levels of government, and leadership is exactly what we showed yesterday."

Trudeau used the word seven times, and Environment Minister Catherine McKenna put it on the record another five times.

This is the federal government's preferred interpretation of its own move on Monday to commit to a national price on carbon.

It was certainly an act that put Trudeau's Liberals out front, to be assailed or commended on their position. And Trudeau and McKenna seemed to come to question period on Tuesday expecting a fight, the prime minister speaking fast and the environment minister attempting a scripted zinger.

"After a decade of failure by the Harper government in meeting its commitments to Canadians," she said after noting the Conservative side's previous support for cap-and-trade, "I guess the only Conservative plan now is to cap all the things they have said and trade it for no plan at all."

Lisa Raitt's Aunt Colleen

McKenna also arrived with authorities and endorsers to cite: Suncor, Shell, the Canadian Wind Energy Association, Stephen Harper's former director of policy, all in favour of pricing carbon. On Wednesday, she added several more: Royal Bank, Tembec, Loblaws, Desjardins, Telus, the Aluminum Association of Canada.

She also rounded on Conservative environment critic Ed Fast, who had apparently offered supportive words for how British Columbia offsets its carbon tax with equal cuts to income taxes.

"I am delighted that he has endorsed the B.C. Liberal government's approach," she said. "Could he let us know what province he does not trust?"

But, more broadly, the Liberals have asserted the principles of protecting the environment and growing the economy.

The Conservatives, meanwhile, have stopped fretting about whether the prime minister was impolite to the premiers on Monday and opted instead to worry about what a national price on carbon might mean for Canadian citizens and consumers, particularly those on fixed and lower incomes.

In this telling, this is about the cost of gas and groceries. Or, to be even more specific, it is about Lisa Raitt's Aunt Colleen.

Competing concerns

The Conservative finance critic mentioned her aunt on Monday, worrying that Liberal action on carbon emissions would make it harder for Colleen, on a fixed income, to afford to pay for gas, electricity and food.

On Tuesday, Raitt continued on with her loved ones, raising first her friend Susan, a divorced mom with two kids in university, who is worried about being able to afford her home.

"My question for the minister is: What does she have to say to my friend Susan?" Raitt asked.

When she stood next and invoked "another friend of mine, Marie," there were grumbles and chuckles from the government side, some members perhaps believing they were witnessing a gambit of some sort.

Raitt was unimpressed.

"It is not funny," she said.

She turned emotional as she explained Marie's situation.

"My friend Marie has three boys," Raitt continued. "Her husband just went on disability. They have a hard time making ends meet. She is trying to deal with it as best she can, but the money is just not there. Now she hears about more taxes."

Marie is worried that she won't be able to afford to keep all the boys in hockey next year.

Costs and benefits

Here could be the crux of the matter.

Trudeau's government defers to each province's freedom to do what it wishes with the revenue from pricing carbon and points to its own efforts to improve social supports. It argues for the economic growth and security that pricing carbon could drive and assure.

And it is likely worth wondering how Marie's situation would differ under a Conservative alternative (presuming that the Conservatives, who still support Canada's current target for reducing greenhouse gas emissions, ever get around to articulating an alternative). Or how everyone's situation would be if we just didn't bother with a significant effort to reduce greenhouse gas emissions.

Environment Minister Catherine McKenna tells the opposition to stop playing politics with the environment in advance of today's vote to endorse the government’s plan to ratify the Paris accord on climate change. 2:18

But ultimately the debate about climate change is about costs and benefits, justifying (and perhaps minimizing) the former and demonstrating (or making the case for) the latter, both for the present and the future. That is the argument the Liberals must now carry.

And if action seems to create a punishing mess in the short term, the public might not be willing to keep a focus on the long term. (In the immediate term, before the action is enacted, there should be economic costing and analysis.)

Speaking to reporters on Tuesday, McKenna twice said this was about our kids and grandkids. In pledging to move forward on Monday, the prime minister invoked his children by name. Raitt has now matched that with three loved ones of her own.

Whether it involves a price or a tax, can the Liberals fashion a response to climate change that works for Xavier, Ella-Grace, and Hadrien, but also for Colleen, Susan and Marie?