With the Conservative Party of Canada in the final days of contesting a fifth federal election, the party is also marking its twelfth anniversary, after the Canadian Alliance and the Progressive Conservative parties agreed to merge on Oct. 16, 2003, giving rise to the Conservative party Canadians know today.

On Oct. 16, 2003, then-PC Leader Peter MacKay and then-Alliance Leader Stephen Harper announced an agreement-in-principle to unite the political right under a single Conservative Party. The PCs and the Alliance were thought to be dividing right-leaning voters, and their merger was aimed at presenting a stronger competitor to take down the Liberals, who held a majority government under Prime Minister Paul Martin.

MacKay and Harper shook hands on the deal and announced it at a joint news conference on Oct. 16.

"I actually had difficulty sleeping last night," Harper said at the news conference. "It's like Christmastime, waking up."

Prior to the merger, the once-powerful PCs were struggling through a period in which they held the fewest seats in party history over three elections. Their majority government had been soundly defeated in the 1993 election, leaving them with only two seats in the House of Commons. They clawed their way up to 20 seats by 1997, but slid back to 12 seats following the 2000 election.

The Canadian Alliance, meanwhile, had only improved marginally over the seat count of its predecessor, the Reform Party, with 66 seats in the House of Commons in the 2000 election.

"This agreement ends vote splitting," MacKay said at the news conference. "It means two plus two can equal more than four. It means winning," he added.

And it did mean winning, if not right away. The merger was ratified on Dec. 5, 2003, and Harper was chosen as the party's leader the following spring. Harper led the Conservatives to win 99 seats in the 2004 election, before seizing a minority government in 2006 and going on to govern for a decade.

At the time of the merger announcement, Diane Ablonczy of the Canadian Alliance was excited about the new union. "It's about time," she told CTV News. "Canada's been suffering because there's one governing party and a bunch of also-rans."

But the merger was not universally accepted among the PC ranks, where some railed against the social conservativism of the Alliance.

MacKay, in particular, faced heavy criticism for allegedly breaking a promise to one of his PC rivals to facilitate the merger. MacKay had been chosen party leader in May of 2003, after brokering a deal with rival leadership candidate David Orchard. Orchard had thrown his support behind MacKay to break a deadlock in the leadership campaign, on the condition that MacKay vow not to merge the PC Party with the Canadian Alliance.

Five months after winning the PC leadership, MacKay was sitting side-by-side with Harper at the merger announcement.

Former PM and PC leader Joe Clark was among MacKay's more vocal critics. "Winding down the Progressive Conservative Party of Canada, which this does, deprives the country of the only other national entity that could form government," Clark said at the time.

Others among the PC ranks were also upset. Andre Bachand, another PC leadership candidate who lost out to MacKay, labelled the merger as "sleeping with the extremists." He resigned from the party before the merger became official, though he later made an unsuccessful attempt to win a seat for the Conservatives in the 2008 election.

MacKay became one of Harper's top cabinet ministers, handling the Foreign Affairs, National Defence and Justice portfolios during his tenure. However, he decided not to run in the 2015 election, and served out his term as Minister of National Defence before stepping away from his MP post.

MacKay says he is turning his attention toward his young family, but he plans to continue actively supporting the Conservative party he helped found.