And, notably, Mulroney was sitting right there in the audience.
In a speech, the current prime minister lauded the merits of both the new Canada-EU deal and the Canada-U.S. pact introduced by his Conservative predecessor, with whom he has had an up-and-down relationship.
Harper noted how analysts made comparisons between the agreements since he announced the EU deal last month.
But right from the start, he reminded his Montreal audience where he stood on the matter: the Canada-European Union deal is bigger.
"Now, I have said it is the biggest deal in our country's history, a claim which I understand has been disputed in some circles," Harper told a crowd of hundreds, including Mulroney, in a speech hosted by the Board of Trade of Metropolitan Montreal.
"So allow me to frame my remarks by addressing this question directly."
The definition of "biggest" is perhaps in the eye of the beholder. Numerous participants have said the new EU pact is deeper, touching more industries and people. But Canada's trade with the EU is still a relative drop in the bucket, at only one-eighth the $740 billion exchanged with the U.S. last year.
Harper pointed out the numerous economic perks of the U.S. treaty, hailing it as "revolutionary" and an "incredible victory for Canada."
Harper credited the 1988 deal with clearing the path for the country's involvement in future economic pacts, such as NAFTA and eventually his government's EU treaty.
He called the deal with the U.S. an "historic event for the world," a first big step toward globalization after the end of the Cold War era.
The agreement, he added, has allowed Canadians to engage in more trade with their next-door neighbours than all EU countries combined.
"(Free trade with the U.S.) has become one of the sturdiest foundations upon which the unprecedented strength of the Canadian economy today is based," he said, before noting that Canadians should be thankful for the work of the Mulroney government.
Harper then proceeded to celebrate some of the benefits he expects from the EU deal.
"Now, the case for why the free-trade deal with Europe is the biggest," said Harper, drawing laughs from the crowd and even letting out a chuckle of his own.
He said Europe's population, at 500 million people, is larger than that of the U.S., making it the largest integrated market in the world.
Harper added that the treaty also includes investments, public markets, services and labour mobility, which he said is why The Economist magazine called the pact a new world model for future commercial accords.
"The agreement with the European Union gives us as Canadians an opportunity, a unique opportunity we have long sought: the ability to lessen our dependence on the American market and to diversify our trade beyond the United States, a trend I should mention that has become evident during the life of the government I lead," he said.
After Harper's appearance, Mulroney was asked by reporters about the prime minister's comparison, particularly his reiteration that the EU deal is the biggest in Canadian history.
"I don't think he said that," Mulroney initially replied with a laugh, before commenting on the free-trade-deal debate: "Well, I'll let others judge that."
Mulroney, who has occasionally had a frosty relationship with Harper, said the prime minister did well with the European deal and he called it a very important achievement.
He then underscored some of the merits of his former government's Canada-U.S. deal.
Mulroney said the U.S. agreement set off an "explosion" of trade of up to $750 billion per year, making it "one of the biggest trade deals between two countries in the history of the world."
He also credited it with creating 4.5 million jobs in Canada, 80 per cent of which are directly linked to free trade.
"Not bad," Mulroney said.
"So, I hope the new treaty will do the same thing. It will be wonderful."