In all the critiques of Peter MacKay's career, what is missing is the most consequential decision the Nova Scotian ever made. It is the one he may regret the most – an ill-fated, horribly handled, backroom deal that seared his credibility, served the old Tory party its final death warrant, and helped secure Stephen Harper's future.

On May 31, 2003, Mr. MacKay won the leadership of the Progressive Conservative Party of Canada. It was done with a vow not to merge with the Canadian Alliance party. And he had every intention of leading the PCs into the next election, then expected within a year's time.

At the time of the leadership convention, his Tories led Mr. Harper's Alliance party in opinion polls. The Alliance had just thrown all its resources into an important Ontario by-election in Perth-Middlesex, and been trounced by the PCs.

Story continues below advertisement

Mr. MacKay looked happily on that result. He was young, popular, attractive. His victory at the convention was expected, as was momentum for his party. The idea that within just a few months' time he would go back on his word and merge the party into a new one, then stay on the sidelines for its leadership race, was far-fetched in the extreme.

But with his convention victory came his "deal with the devil," as then-Conservative party strategist Goldy Hyder called it. It changed everything. Mr. MacKay felt that to win the final leadership ballot against Alberta's Jim Prentice, he needed the support of David Orchard, the anti-free-trade maverick from Saskatchewan. Mr. MacKay went to Mr. Orchard and, in a written pact, promised not to merge with the Alliance and also to revisit the North American free-trade agreement. When the news media found out, an uproar ensued. The deal was denounced as deceitful, Machiavellian, a Faustian bargain. Mr. MacKay was mauled in a media scrum. "It was like trying to swim in a shark tank," he later recalled.

Instead of leaving the convention in full stride, the party and Mr. MacKay went out limping. Mr. Harper was handed a gift as big as the Liberal sponsorship scandal. The old Tories were now ripe for the picking.

Deal making is hardly a new thing at political conventions. Backroom bargaining is common. But the way it is handled is critical. As Bob Plamondon recounts in his 2006 book Full Circle, the MacKay people had no communications plan to sell the Orchard deal. They could have been open about it, spun it as coalition building, as reaching out to all sides. Instead they kept the written agreement sealed at the convention, stoking suspicion that it was a dirty deal.

With hopes of rebuilding the Tories shattered, Mr. MacKay then sought out the merger with the Alliance. For the old Tories, it wasn't a merger so much as an Alliance takeover. With so many more seats in the House of Commons, the Alliance side had the cards. With his credibility damaged, Mr. MacKay decided not to contest the leadership of the resulting new Conservative Party of Canada, which Mr. Harper easily won.

Although Mr. MacKay's options had become limited, he deserves credit for his role in unifying the political right and serving in the role of faithful soldier to the party and its leader thereafter. But old Tories who hoped that he would uphold the progressive banner have surely been disappointed. When they look at the hard line he followed as a cabinet minister, in foreign affairs, defence and especially the justice portfolio, they can well wonder if Mr. MacKay was really a progressive in the first place.

He had charm and was well-liked by those who knew him. But in his various portfolios, he repeatedly stumbled upon controversies – the purchase of F-35s, the military-helicopter fishing-trip junket, judicial appointments, drafting unconstitutional laws – that soiled his record.

Story continues below advertisement

Mr. MacKay's legacy is as a terminator of one conservative party and a builder of another. Were it not for the Orchard blunder, he would have certainly led the PCs into another election. Different chapters in our political history would have been written.