A high-profile Liberal MP is the target of criticism after he abstained from the House of Commons vote on Canada’s joining U.S.-led airstrikes against ISIS in Iraq.

When MPs passed the motion Tuesday night, Liberal MP Irwin Cotler was notably absent.

Cotler had called for airstrikes in Syria three years ago, and has since blamed international inaction for allowing ISIS to develop. But now, he says, he couldn't vote to support the mission as proposed.

"The government's motion lacks clarity about what the strategic nature and limits of what Canada's mission will be," Cotler wrote in a posted online ahead of the vote.

Cotler added that he has always been a proponent of the Responsibility to Protect doctrine, which obliges the international community to intervene when war crimes have been committed.

Cotler also said that he is “deeply disturbed†by Prime Minister Stephen Harper’s statement that Canada would seek approval of the Syrian President Bashar Assad to carry out operations in Syria.

“To allow the perpetrator of war crimes, crimes against humanity, ethnic cleansing, and genocide, to green-light Canadian intervention is to turn (Responsibility to Protect) on its head,†Cotler wrote.

NDP Leader Tom Mulcair said Cotler’s explanation is the “tip of the iceberg†on the lack of consensus within the federal Liberal party.

"What it does show to me, is perhaps for the first time an explanation of why the Liberals keep flip flopping on this issue," Mulcair said after his party caucus met Wednesday.

"It is because there is no consensus within their caucus," Mulcair said, noting that the NDP ranks all voted against the mission.

Tory minister Jason Kenney aggressively slammed Cotler for his decision:

Liberal Leader Justin Trudeau defended his party’s stance against the mission.

"We feel Canada does have a role to play in the push against ISIL, however we could not support the government's plan last night. We have a different idea of what is necessary."

Trudeau said Cotler has been calling for airstrikes in the region for three years, but he can't support the government's decision.

The motion passed 157-134. All 155 Conservative MPs, Green MP Bruce Hyer and Independent MP Brent Rathgeber voted in favour of joining the mission.

Cotler, who represents the Montreal riding of Mount Royal, has said that he will not run in next year's federal election.