Bloc Quebecois Leader Gilles Duceppe has deputized himself as Stephen Harper's truth cop.

With less skin in the game than other leaders, Duceppe may be in the best position to take the prime minister to task over his repeated pounding on the possibility of a coalition of opposition parties forming a government after the election.

Duceppe said he never intended to go into great detail about the opposition discussions in 2004 aimed at toppling Paul Martin's Liberals and installing Harper as prime minister. The Conservative leader's assertion that any government led by a party without the most seats would be "illegitimate" forced the separatist chief to change his mind.

Harper invited him to take part in those discussions. Harper asked for his input in a Conservative budget and throne speech. So as far as Duceppe is concerned, there is only one conclusion that can be drawn -- Harper "is lying."

The Bloc leader has admitted he has little to lose from the issue -- he makes no secret he is willing to use his position to win concessions for Quebec with any party.

He was willing to do so with Harper in 2004, with then-Liberal leader Stephane Dion in 2008, and likely today with whoever emerges with cards to deal after this election.

But it's a incendiary issue for the Liberals and their leader, Michael Ignatieff. He had to use the first day of campaigning Saturday to disavow any notion of a coalition in unequivocal terms.

And it has the potential of being a winning issue for Harper, especially when the coalition can be couched as a scary alliance of "arch-centralists and Quebec sovereignists." It also makes the election about the opposition rather than on his own government's record.

On Sunday Harper was back at it at a rally in Brampton, Ont., ominously warning that if he didn't win a majority, Ignatieff would "move with lightning speed" to team up with the NDP and Bloc.

Harper said he would never consider trying to govern from second place.

That was the last straw for Duceppe. In an unusually lengthy and angry dissertation of the events of 2004, when Harper was the leader of the Opposition, Duceppe said Harper tried to do everything he now accuses the opposition parties of contemplating.

He read a letter the three had sent the governor general of the day, Adrienne Clarkson, asking that she "consider all your options" should the Liberal minority government of Paul Martin fall.

There were only two options, Duceppe noted Sunday -- an election or ask Harper if he could get the confidence of the House, meaning the NDP and Bloc. Then he looked at the bottom of the letter for effect.

"I remember I signed it, Gilles Duceppe. And Jack Layton signed it. And the other was? Stephen Harper!"

Duceppe said it was clear then that Harper was attempting to become prime minister after having lost the election months before.

"He would have had to present a speech from the throne that received our support. He would have had to compromise and I believe he was prepared to do that in 2004," Duceppe said.

"It's clear what his intention was. As they say in English, it smells like a dog, it looks like a dog, most probably it's a dog."

Duceppe added that Harper proposed the three leaders co-operate on strategy, as well.

One proposal was not to ask any questions on the just tabled Liberal budget, and instead focus on attacking Martin over the sponsorship scandal.

That's also the way Layton remembered it. Harper was trying to become prime minister even though he had recently fallen short of unseating Martin, Layton told reporters Sunday.

"(The letter to the governor general) was designed to illustrate that such an option is legitimate in Canadian constitutional traditions and there was no question about it, I was in the meetings where this was discussed."

"It was his (Harper's) proposal," he added.

The Conservatives sent out a release Sunday afternoon denying the 2004 actions were an attempt to form a coalition. Harper described it as a "co-operative effort."

In an interview with CTV Toronto's Ken Shaw, Harper had an entirely different recollection of what he was trying to accomplish in 2004.

"It was Mr. Martin who was threatening an election, who was claiming that I was trying to move non-confidence in the government when I was not," he said.

"Had the governor general asked me, my position would have been, Ken, very simply that there's not an issue of confidence here, we're not trying to bring the government down, these are small matters and we think we can work a compromise. And, by the way, that's exactly what we did."