Is it time for a coalition of the left in Canada? There's probably no one better to ask than Bob Rae.

As acting leader of the federal Liberals and former leader of the Ontario NDP, he has watched both parties lose power to conservative governments.

It's also a question former prime minister Jean Chretien has asked, after seeing his once mighty Liberals reduced to third-party status.

Speaking to Kevin Newman on CTV's Question Period in the final days of his stewardship of the Liberal party, Rae rejected the merger outright.

"Well, I think if the logic was really that compelling, Mr. Chretien would have done it in the 13 years he was leader of the party," Rae said. "…if he really wanted to try and effect a merger and bring people into cabinet he could have done that, but he didn't do it."

As often as leaders cross the floor and as much as the numbers make sense, there's simply no way to bridge the ideological divide between the parties, he said - even as the NDP continues to lean further to the centre. New Democrats have recently considered reducing the number of references to socialism in their constitution.

"I don't think any leader can say, you know, never but my view is just that a whole lot of things would have to happen and I don't see any sign of it happening," Rae said.

"They're two different parties, and two different ways of looking at things."

The Liberal party, in Rae's mind, is committed to pragmatism and rejects ideology. The NDP, on the other hand, is more "faith-based" - it's part of the reason he left the party in the first place.

"They're based on a deep political faith, on a deep political ideology," he said. "A lot of their policies (aren't based) on reason, but on their analysis of what should work."

A merger simply wouldn't work, Rae said, because "you don't know what the consequences of that will be. You don't know how many people on either side of the equation would say, 'No, I don't want to be here.'"

Rae will be stepping down as acting leader of the Liberals after the party chooses a new leader next week.

Watch the full interview with Bob Rae live on CTV's Question Period, 11 a.m. ET on CTV. (Check local listings).