Speaking in Quebec City today, Harper was asked about Justin Trudeau's recent pledge to end Canada's bombing mission in Iraq and Syria and normalize relations with Iran.
Harper seized on the opportunity, accusing the Liberal leader of planning to become "best friends" with Iran, a state sponsor of terrorism.
He went on to lump Trudeau together with NDP Leader Tom Mulcair, whose party has long stood in opposition to the mission in Iraq and Syria.
Harper also noted that Trudeau made his comments on the anniversary of the Air India disaster, a day now dedicated to remembering the victims of terrorism.
On that day, Trudeau told CBC that there are "ways to re-engage" with Iran, and that in Iraq and Syria, Canada would be more effective in a military training role, similar to the one it played in Afghanistan.
"I think it's very strange to say that his priority is the restoration, or to become best friends with one of the state sponsors of terrorism in the world, the government of Iran," Harper said.
"And that he wants to cut the relationship we've established with all of our allies and with a large international coalition to take the fight against the so-called Islamic State in Iraq and Syria, a group that has executed and is planning attacks against Canada and Canadians.
"I think on behalf of both Justin Trudeau and Thomas Mulcair, their positions on the military mission in Iraq and Syria, their mission against ISIS, is irresponsible electoral politics."
In a statement, Liberal House leader Dominic LeBlanc assailed Harper for making "divisive and hyper-partisan foreign policy" his trademark and insisted Liberals are committed to fighting Islamic State militants.
"We can — and should — be training away from the front lines, as our allies have been doing. A well-funded international humanitarian aid effort is also needed, and Liberals have called on the federal government to expand the target number of Syrian refugees to Canada to 25,000," LeBlanc said.
"Instead, Mr. Harper has committed us to an ill-defined combat mission with no concrete objectives and no exit strategy."