Prime Minister Paul Martin hammered on the theme of national unity as his campaign swept through Quebec on Wednesday, but couldn't put the sponsorship scandal behind him.

"National unity is an issue in this election," said the Liberal leader. "(Bloc Leader) Gilles Duceppe formed a pact with [new Parti Québécois Leader] Andrew Boisclair. They said this election is the first step on the way to a referendum. Gilles Duceppe will put it on the table. What we're saying (is), we're going to defend Canada.

"I can tell you right now we're not going to allow the separatists to divide Quebec families," added Martin. "We're not going to allow separatists to divide this country."

But Martin's message got curtailed with news that former prime minister Jean Chrétien has launched a legal challenge against the Gomery report on the federal sponsorship inquiry, saying the findings aren't supported by the evidence.

Asked by reporters about Chretien's move, Martin said his predecessor has a right to fight the decision, but that the Liberals support Gomery's conclusions.

The sponsorship program was designed to raise the federal government's profile in Quebec after sovereigntists narrowly lost the 1995 separation referendum.

Justice John Gomery's report concluded that Liberal politicians mismanaged the program, and some party organizers in Quebec sought and accepted donations from ad companies that had received lucrative government contracts starting in the mid-1990s.

The Bloc's popularity has risen since the revelations of the sponsorship scandal.

To boost their sagging popularity, the Liberals introduced Marc Garneau, the first Canadian astronaut in space, as their candidate in Vaudreuil-Soulanges, a rural riding between Ottawa and Montreal currently held by the Bloc Québécois.

Martin used Garneau's candidacy to pump up his campaign theme of touting Canada's economic performance, and wondered if that's what sovereigntists want to separate from.

Duceppe, who laid out his platform forcusing on an independent Quebec, shrugged off the Liberal strategy to play up national unity to win votes.

"They have their own strategy. We have ours and we'll face them."

While campaigning in Quebec's capital, Conservative Leader Stephen Harper said the referendum is not an issue.

"All the BQ and Mr. Martin want to talk about is referendum, a referendum that nobody wants and that we're not having," said Harper.

"I think Quebec is entitled to a more substantial debate about how we can make federalism work and how we can advance the interests of Quebecers in this country."