On Wednesday, Peladeau said during a PQ leadership debate that immigration was hurting the sovereignty movement.
"We don't have 25 years ahead of us to achieve it," Peladeau said. "With demographics, with immigration, we're definitely losing one riding each year."
Couillard said the comments signalled "a clear shift toward ethnic nationalism" in the party.
"Since the charter (the values charter the PQ wanted to implement) there has been a very unfortunate drift," Couillard said at the legislature.
"There is no longer a financial argument or an economic argument for the separation of Quebec so they’re clinging to whatever they can."
Peladeau said after the debate he wasn't attacking immigrants but rather demonstrating the importance of reaching out to them in the same way the federal government does.
He reiterated that immigration is an asset that has benefited Quebec.
Peladeau is the perceived front-runner in the PQ leadership race to be decided in May.
Some of the other hopefuls were ill at ease with Peladeau's comments and distanced themselves.
But Couillard said the party itself and its leadership should also have distanced themselves from the media mogul's comments.
Other members of the governing Liberals also took aim at Peladeau's comments.
Health Minister Gaetan Barrette made a parallel between the PQ and the Front National, a far-right party in France.
Intergovernmental Affairs Minister Jean-Marc Fournier suggested the opposition party should change its name.
"It's the party of certain Quebecers, not all Quebecers," Fournier said, saying the party co-founded by Rene Levesque is now unrecognizable.
"They love to talk about values — it is a value to classify Quebecers?" Fournier asked. "Democracy is a place for everybody."
