The Liberal prime minister had campaigned a year earlier on replacing the Red Ensign — the country's unofficial flag which paired Canada's coat of arms and the Union Jack — with a uniquely Canadian emblem as the 1967 centennial approached.
As Canada prepares to celebrate its now-beloved flag's 50th birthday on Sunday, the bitter national brawl that erupted as Pearson forged ahead with his plans is a distant memory.
"It was the debate, the issue," said Conservative Sen. Marjory LeBreton, who worked in opposition leader John Diefenbaker's office as the battle raged on.
In a recent interview, LeBreton pointed out that Canada's military veterans had fought under the Red Ensign in two world wars — one of which had only ended 20 years earlier.
Nonetheless, young Canadians argued with their parents about the need for a new flag; one would-be designer even submitted a flag proposal festooned with John, Paul, George and Ringo as Beatlemania gripped North America.
Some English-Canadians, meantime, accused French-Canadians — the aforementioned "pea soupers" — of being behind the push to cut Canada's ties to Great Britain even as one Quebec MP, Pierre Trudeau, sniffed that Quebecers didn't give a "tinker's damn" about a new flag.
Just a few months after Pearson's PR disaster in Winnipeg, however, he managed to convince a majority of parliamentarians — including Conservatives — to vote in favour of the red Maple Leaf chosen by his flag selection committee as the new official emblem.
It wasn't the flag he liked best and had displayed to the angry legion members that summer. The so-called Pearson Pennant, a sprig of three red maple leaves bordered by blue bars, ultimately lost out to the flag Canadians know today.
But the prime minister was elated nevertheless as the Maple Leaf was hoisted over Parliament Hill on Feb. 15, 1965. Diefenbaker, on the other hand, wept.
Even some of Dief's staff members didn't get what all the fuss was about as they spent months reading through "thousands and thousands and thousands of letters" sent to their boss urging him to save the Red Ensign, LeBreton recalled.
"I personally wasn't troubled by the idea of a distinctive Canadian flag," said the senator, who was 25 at the time and had a secret fondness for the Pearson Pennant.
Diefenbaker's personal secretary, Bunny Pound, however, whooped it up when the new flag was raised on the Peace Tower.
"She was wildly cheering this new flag and Mr. Diefenbaker was shedding a tear. And he looked down at Bunny Pound and he just scowled at her ... and he didn't speak to Bunny Pound for about two months after that," LeBreton said with a laugh.
Roy Mayer was a young advertising executive that day and recalls flying two of the new flags out of his downtown Ottawa office window and being met with a chilly reception unrelated to the weather.
"There were no Canadian flags flying anywhere except on Parliament Hill that day, and I got nothing but ridicule and derision when I flew those flags," said Mayer, 74, who's long advocated for a federal flag day holiday on Feb. 15.
Such antipathy was nowhere in sight by the 25th birthday, when Conservative prime minister Brian Mulroney spoke glowingly of the Maple Leaf as he presided over a Parliament Hill ceremony attended by hundreds of schoolchildren.
There has been no official word from Prime Minister Stephen Harper's office about his plans to mark the 50th birthday on Sunday. Gov. Gen. David Johnston will preside over an Ottawa ceremony and unveil a commemorative coin and stamp.
While some Conservative MPs, including Tony Clement, have been tweeting about the birthday, the government has been criticized by flag advocates for paying short shrift to the milestone. Mayer and other self-described flag-wavers wrote a letter to Harper last month to complain.
Canadian Heritage has spent just $50,000 to mark the occasion. That's compared to almost $10 million spent on the 200th anniversary of Sir John A. Macdonald's birth and the bicentennial of the War of 1812.
Some suspect partisanship, pointing out that the Maple Leaf was a Liberal initiative, as was the Charter of Rights and Freedoms — another milestone the Conservatives were accused of snubbing three years ago, on its 30th anniversary.
"This government has gotten a reputation for partisanship in just about everything it does," Liberal Leader Justin Trudeau said recently, adding that if he were prime minister he would celebrate national milestones regardless of the party behind them.
"Fifty years of the Canadian flag is an extremely important milestone for our identity and our national pride, and I certainly hope the prime minister does a good job of celebrating something that binds Canadians together so well."
Trudeau and former prime minister Jean Chretien will be in Mississauga, Ont., on Sunday to celebrate the birthday.
Follow Lee-Anne Goodman on Twitter @leeanne25