Parizeau, who nearly led Quebec to independence in 1995, lent his clout to the campaign of Jean-Martin Aussant, head of Option nationale.
The new party announced on Saturday that Parizeau donated $200 to help Aussant in his own riding.
The development comes as the Parti Quebecois, which has been leading in the polls, tries to shore up support to secure a majority mandate in the Sept. 4 election. Option nationale has been polling at around two per cent provincewide.
Marois said she has "the utmost respect for Parizeau" but urged "sovereigntists and progressives" to unite under the PQ banner.
"I want to tell Quebecers that are listening, if you want to get back to the goal of creating a country, only a majority government can do it, a sovereigntist government of the Parti Quebecois," Marois said at a news conference Saturday.
Option nationale, which presents itself as more committed to Quebec independence than the PQ, already had the backing of Parizeau's wife, Lisette Lapointe.
Lapointe cautioned on Twitter that Parizeau's support for Aussant shouldn't be interpreted as a rejection of his former party, but rather an attempt to get him a seat in the provincial legislature.
Aussant left the PQ to sit as an independent in 2011 over differences with leader Pauline Marois. He founded Option nationale a few months later.
Aussant, who has outlined a game plan to make Quebec independent within his party's first mandate, said Parizeau's support is "a clear expression of the quality of the work we've been doing."
"I think Mr. Parizeau believes we are doing some very efficient work toward sovereignty," Aussant said in an interview, describing the former premier as his political mentor.
"We are the only party right now with a clear message toward making Quebec a country."
Parizeau, 82, has criticized Marois before.
In a speech last summer he took jabs at Marois for not doing enough to promote independence.
Marois hasn't committed to a timeline for holding a referendum if the Parti Quebecois win the next election. She faced criticism this week for backtracking on a party pledge to automatically initiate a referendum if 850,000 Quebecers sign a petition.
As Quebec's premier, Parizeau led the province to within a few votes of independence in 1995, blaming "money and the ethnic vote" for the loss.
With Parizeau signalling his support for Aussant, the PQ no longer has the clear endorsement of any of the three sovereigntist leaders in the 1995 referendum.
Lucien Bouchard, who campaigned alongside Parizeau as head of the Bloc Quebecois, backed away from the party years ago. Ex-Action democratique du Quebec chief Mario Dumont, another prominent voice for the "Yes" side during the referendum, also stopped promoting sovereignty.
Charest took aim at Coalition Leader Francois Legault's plan to trim the province's union membership, saying it would cost millions in taxpayer money to pay them off.
Legault, meanwhile, tried to persuade voters his plan to "clean up" corruption in Quebec was achievable.
"I may not be a big talker like Jean Charest, but I'm a guy who delivers the goods," he said.
- with files from Stephanie Marin, Martin Ouellet and Alexandre Robillard
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