Liberal Leader Michael Ignatieff says he would be willing to form a government according to parliamentary rules if Stephen Harper wins the most seats in the election but fails to win the confidence of the House of Commons.

"If the Governor General wants to call on other parties, or myself, for example, to try and form a government, then we try to form a government," Ignatieff told CBC's Peter Mansbridge in an exclusive interview Tuesday afternoon.

"That's exactly how the rules work and what I'm trying to say to Canadians is, I understand the rules, I respect the rules, I will follow them to the letter and I'm not going to form a coalition. What I'm prepared to do is talk to Mr. Layton or Mr. Duceppe or even Mr. Harper and say, 'We have an issue, and here's the plan that I want to put before Parliament, this is the budget I would bring in,' and then we take it from there."

The statement is the closest Ignatieff has come to describing how he could become prime minister since his vow on the first day of the campaign renouncing a coalition with the NDP and the Bloc Québécois.

Ignatieff, who had signed on to form a coalition with the NDP supported by the Bloc when then-Liberal leader Stéphane Dion had worked out an arrangement three years ago, told Mansbridge he later rejected the idea because it wasn't in the interest of Canadians.

"I could be sitting here and talking to you as prime minister and chose not to do that because I felt that it wasn't in the national interest," he said. "So I rejected the coalition in 2008 and I've gone into this election in 2011 firmly excluding the coalition.

"Mr. Harper had discussions about a coalition in 2004 with Mr. Duceppe and Mr. Layton. Not me."

Shortly after the interview, the Tories shot back, issuing a statement that said Ignatieff "made clear this is his agenda to become prime minister, even if he loses the election."

"Michael Ignatieff’s ambition to be prime minister trumps everything: our economic recovery, what‘s best for families, even how Canadians vote," the Conservatives said.

Denies negative campaign

Ignatieff also denied he has started running a negative campaign against Harper, saying the question boils down to whether you can trust the Conservative leader with power. The Liberals were forced to amend an anti-Harper attack ad on health care after it was determined a quote was wrongly attributed to Harper.

"I think that we have to be tough, I think we have to frame up the issues. I think that Canadians want to know what the choices are on May 2," Ignatieff said of the ads his party is running.

"We feel a strong campaign here is essential."

Mansbridge pressed Ignatieff about some of the campaign ads the Liberals are running, specifically ones that say Stephen Harper wants "absolute power."

Ignatieff said Harper shut down Parliament twice and has been held in contempt of Parliament. He said the important question of the campaign is: "Can you trust this guy with power?"

"If you can't trust this man with democracy, you can't trust him with anything else," Ignatieff said.

Ignatieff's sit-down discussion with Mansbridge is part of a series of one-on-one interviews with the party leaders. Harper has agreed to an interview at a time that has yet to be determined.

During the interview, Ignatieff brushed off any similarities his party's platform may have with the NDP, saying that unlike the Liberals, the NDP's costing table does not add up.

But the NDP was quick to offer a rebuttal, saying their platform is not only costed, but also third-party validated.

"Unlike the Liberals’ platform, it doesn’t make multi-billion uncosted promises (e.g., HST payment to Québec, prescription drugs, high speed rail), nor does it ignore two years," the New Democrats said in a statement.

"And unlike the Conservatives, it doesn’t contain $11 billion of unidentified cuts."