The news came as the Parti Québécois, which has been leading in the polls, tried 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 Québécois," Marois said at a news conference Saturday.
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.
Parizeau has criticized Marois before, including in a speech in Montreal last summer where 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 Québécois 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.
With the 82-year-old 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 Québécois, backed away from the party years ago.
Former Action démocratique du Québec chief Mario Dumont, another prominent voice for the "Yes" side during the referendum, also stopped promoting sovereignty.
Later in the day, Coalition Avenir Québec leader François Legault also commented on Parizeau's endorsement of Aussant.
"You can't underestimate the PQ's capacity to create new psychodramas," he said.
Donation should not be seen as rejecting PQ: Lapointe
After Aussant tweeted a mention of Parizeau's donation on Saturday, Parizeau's wife Lisette Lapointe then confirmed his tweet with one of her own, saying the PQ elder statesman hopes to see Aussant keep his seat in the National Assembly.
Lapointe said the donation is in support of Aussant's campaign to represent the riding and should not be interpreted as a rejection of the PQ.
Lapointe herself was canvassing for Aussant in the Nicolet-Bécancour riding on Saturday.
"The backing from Mr. Parizeau is to be interpreted more as an individual backing in my case than a rejection of a whole party and the backing of a new one. So I think he wants my re-election in Bécancour, and that's very much appreciated because the race is very tight here," said Aussant.
Aussant left the PQ in 2011 to create his own party after concluding that Pauline Marois' stance on sovereignty was too soft.
The electoral race in Nicolet-Bécancour is expected to come down to a final sprint between the Coalition Avenir Québec's Donald Martel and Aussant.
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