Duceppe told a news conference on Wednesday that Quebec's voice in Ottawa has weakened under the NDP and only a strong Bloc presence can change that.
"We are better represented when we're represented by people thinking like us," Duceppe said, making his return official at Bloc headquarters in Montreal. "Democratically elected and speaking the way we want them to speak, defending the interests which are our interests."
He advised the NDP to focus its efforts elsewhere: "Let us beat Harper in Quebec and do your job in the rest of Canada — that would be of a great help."
But NDP Leader Tom Mulcair said Wednesday that after four years, Quebecers still have a positive view of his party and want positive change.
"They want something moving forward," he told reporters in Ottawa. "They don't want to play as extras in Back to the Future 4."
Duceppe, 67, faces the monumental task of getting the moribund political party ready for the upcoming federal election in a matter of months.
The Bloc currently has just two MPs out of 75 in Quebec.
The veteran politician takes over from Mario Beaulieu, whose tenure was marked by strife within the party.
But Duceppe made it clear it was about working with Beaulieu and not pushing him aside.
The two men had a falling-out in 2014 after Beaulieu said it was time to put an end to what he called 20 years of go-slow approach to sovereignty.
Duceppe noted the party is in good financial shape, has 20,000 members and an association in all 78 Quebec ridings, although only about one-third of candidates have been selected.
"The gesture we're making today isn't Duceppe replacing Beaulieu, it's Duceppe joining Beaulieu and the team in place and those who will come," Duceppe said. "We're adding, we joining together and we'll move forward together."
Duceppe invited sovereigntists of all stripes to rally under the Bloc banner.
Beaulieu, a hardline sovereigntist who was previously head of the Societe Saint-Jean-Baptiste de Montreal, won the Bloc leadership race nearly one year ago on a sovereignty-first platform.
But the political novice concluded he was running out of time with this fall's federal election looming.
Enter Duceppe, who Beaulieu said will give the Bloc campaign an experienced leader and a name recognition he doesn't have.
"I came to the conclusion that we needed to give new breath to the Bloc campaign," Beaulieu said. "I'm convinced that with Gilles Duceppe as leader, we are headed towards a new victory for the Bloc."
Beaulieu will stay on as party president.
Duceppe said he decided reprise a roll he held for 14 years after a discussion with Parti Quebecois Leader Pierre Karl Peladeau, who assured him the PQ is ready to provide the Bloc with support.
Duceppe became leader in 1997 and under his stewardship, the party won 44, 38, 54, 51 and 49 seats before the 2011 meltdown.
The party won a mere four seats that year as Jack Layton's NDP roared through the province.
Among the casualties included Duceppe, who lost the Montreal riding he had held since winning a byelection in 1990.
Duceppe said he hasn't chosen a riding yet.
Mulcair said the Bloc's objective is to replace the NDP in Quebec. But he said his party is committed to replacing "division and fear with hope and optimism."
Denis Lebel, the Conservatives' Quebec lieutenant, said that prior to 2011, the Bloc elected large teams of MPs who did little for Quebec.
"It's not a question of who the leader of the Bloc is," Lebel said in Ottawa. "We work in Ottawa to make Quebec a very strong province in a united Canada and for me, that's what important."
Liberal Leader Justin Trudeau told reporters his party will offer a direct contrast to the Bloc's desire to separate Quebec from Canada.
"There's going to be some very serious debates about the future of the country from all different perspectives and I think that's a good thing for Canadians to have strong competing visions of where this country needs to go," Trudeau said.
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