NDP Leader Tom Mulcair says the timing of Harper's visit to the Vatican is fortuitous, coming just one week after the Truth and Reconciliation Commission called on the pope to travel to Canada to issue an apology.
Mulcair says Harper should ask Pope Francis if he'd be willing to do so.
Harper is scheduled to meet the pope on Thursday, as he wraps up a whirlwind trip to Ukraine, Germany for the G-7 summit, Poland and Rome.
Church officials in Canada have in the past apologized for the abuse suffered by thousands of aboriginal children in church-run residential schools, as have the United, Anglican and Presbyterian churches.
But Justice Murray Sinclair, who headed the just-concluded Truth and Reconciliation Commission, says the Pope is the "spiritual and moral leader" of the church and residential school survivors are disappointed that he has not yet made a "clear and emphatic public apology" in Canada.
In an interview Sunday with CTV's Question Period, Mulcair noted that Pope Francis' predecessor, Benedict, formally apologized for the abuse of children in church-run schools in Ireland.
"With all the evidence that's now on the table, the Vatican should issue a formal apology for the Catholic Church's role in the residential schools," Mulcair said.
"While the prime minister is with the pope, he should simply ask him if he's willing to issue that sort of an apology. That's something constructive that's being asked for that we could do."
Aboriginal Affairs Minister Bernard Valcourt's office said Sunday that the minister has written to the Vatican — as well as to provinces, territories and the Federation of Canadian Municipalities — to bring to their attention the commission's report and 94 recommendations.
Mulcair said it's unrealistic to promise to implement all of the recommendations, as Liberal Leader Justin Trudeau has done.
An NDP government would "sit down with First Nations; we'll prioritize, we'll get to the subjects that they consider the most important and we'll do it in (the) order that they consider the most important," he said.
"It's not a matter of snapping your fingers and saying that you're going to do all 94 at once. That's not realistic and it's not going to happen."
Among other things, the commission has called for an overhaul of education, corrections and child welfare systems.
Mulcair noted that some of the recommendations require collaboration with the provinces and territories. He said an NDP government would "start with things that are easy to tackle."
Also on HuffPost