Dalton McGuinty won't be challenging Justin Trudeau for the federal Liberal leadership.

The Ontario premier has decided to look for challenges outside politics altogether.

"I made a commitment…both to myself and to some supporters to carefully consider a run for the Liberal leadership for the Liberal Party of Canada," McGuinty told The Canadian Press.

"I've talked it over with family and supporters and I've decided not to run."

McGuinty has been under mounting pressure to run for the federal party's top job since he shocked the nation last week by announcing his intention to resign as premier.

Some of his closest advisers have put together a campaign team that would have been ready to go this week had McGuinty decided to take the plunge.

'I would argue [the federal leadership] takes a decade-long commitment and that is not something that I'm prepared to commit to after my time in politics.'—Dalton McGuinty

Liberals eager for a heavyweight rival to Trudeau, the prohibitive front-runner, have been urging McGuinty to leap to the federal arena. A source close to the premier said he's received encouragement from "Bay Street titans, private labour leaders, student and community leaders, particularly by current mayors, former premiers from across the country and two former prime ministers."

McGuinty thanked all those who would have supported his candidacy and joked that his wife, Terri, "thanks all those who were opposed."

He said two factors heavily influenced his decision to stay out of the fray: the conflicting timing of the federal and provincial leadership races and the 10-year commitment required to rebuild the shattered federal party, which was reduced to a 34-seat rump in the 2011 election.

The Ontario Liberal Party has set the weekend of Jan. 25 for choosing his successor. McGuinty noted he's promised to stick around as premier until his successor is chosen, "a job that takes 110 per cent of my time." That would leave little time to campaign in the federal race, which is already well underway, although the starting gun doesn't officially go off until Nov. 14, and culminates on April 14.

Moreover, McGuinty noted he's served 22 years in provincial politics — 16 at the helm of Ontario Liberals, nine as premier — and said that's enough for him, and for his family.

"My family has supported me throughout. So, it's time for me to take on challenges outside of politics and I am confident that I'm going to find other ways to serve my province and my country and that's what I look forward to doing."

The 57-year-old, who spent years turning around the provincial Liberals' dismal prospects, said he's simply not prepared to make the kind of long-term commitment required to rebuild the federal party.

"It is a big undertaking. It is a long-term commitment. It reminds me somewhat of where our party (in Ontario) was many years ago," he said.

"I would argue it takes a decade-long commitment and that is not something that I'm prepared to commit to after my time in politics."

'Tremendous bedrock of goodwill'

That said, McGuinty said he is "very optimistic" about the future of the federal Liberal party.

"One of the things I've discovered in talking to people in different parts of the country is there's a tremendous bedrock of goodwill and enthusiasm. It's not being manifested on the surface at this point in time but it is there nonetheless," he said.

"I would also argue…that we are, by inclination and in terms of our history, we are small 'l' liberals, we Canadians. And it's just a matter now of doing the necessary work to recommit ourselves to Canadians, to show people that we are hungry, that we have good ideas, that we understand the future, we know where our place for success can be found in that future and to get on with the work.

"So I am, in fact, very optimistic about the future of my federal party."

The prospect of an effective coronation for Trudeau, eldest son of former prime minister Pierre Trudeau, fuelled much of the movement to draft McGuinty. Even those who support the Montreal MP, the federal party's undisputed rock star, believe it would be better for the party to have a vigorous race that attracts public attention and tests the mettle of the eventual winner.

McGuinty said he believes "it's always better that we have a strongly contested race for the leadership, that's just healthier for all concerned."

Still, he added: "Anybody who seeks the leadership here, I think my advice to them would be this had better be about much bigger things than just stopping another candidate. You know, you've got to have some ideas and some energy and some idealism that you want to put on the table."

Other than Trudeau, a handful of little-known contenders are already stumping the country: constitutional lawyer Deborah Coyne, mother of Trudeau's half sister, Ottawa lawyer David Bertschi and Vancouver lawyer Alex Burton. Several others appear close to jumping in, including Liberal House leader Marc Garneau, Canada's first astronaut; former MP and leadership contestant Martha Hall Findlay and Toronto lawyer George Takach.