Bob Rae has officially retired from federal politics, opening up his downtown Toronto seat to either an influx of fresh blood in Justin Trudeau's Liberal caucus or an NDP upset in an upcoming by-election.

Liberal supporters are gearing up for the party nomination in the traditional stronghold of Toronto Centre – with the focus turning to the possible candidacy of former CTV Canada AM co-host Seamus O'Regan.

Mr. O'Regan is a close friend of the Liberal Leader, and sources say they have spoken about the possibility of him seeking the seat.

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Mr. O'Regan cut his political teeth in former Newfoundland and Labrador premier Brian Tobin's office, where he served as a speechwriter and strategist before becoming a television celebrity.

Mr. Trudeau is widely viewed as a harbinger of change for his party, having banished the old guard and brought the Liberals both generational renewal and an end to infighting.

If his poll numbers are to be believed, he has the potential to attract new supporters – and candidates – from across the country.

Mr. O'Regan, now a Toronto resident, has lately been filling in on Astral radio's Newstalk 1010. He refused to comment on the speculation about a possible Liberal candidacy.

If he runs, Mr. O'Regan would first have to win the nomination in the riding. Mr. Trudeau has committed to open nominations and pledged not to appoint anyone, though he could well provide his support to star candidates he attracts to the party in key ridings.

An emotional Mr. Rae said Wednesday he will soon turn 65 and has no plans to return to politics. His exit comes as the House of Commons adjourns for the summer, a time when politicians return to their constituencies and parties take stock.

However, NDP officials said they have also been getting ready for Mr. Rae's retirement and plan to put up a fight in the riding. The party finished in second place there with 30 per cent of the vote in the 2011 election, losing by 6,000 votes to Mr. Rae, who took 41 per cent.

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NDP national director Nathan Rotman said the by-election, to be called in the next six months, is the Liberals' to lose. Still, he said the NDP riding association in Toronto Centre is strong, noting the New Democrats hold two adjoining ridings.

"We are looking to build on our strengths in the last election campaign," Mr. Rotman said.

The by-election will be Mr. Trudeau's first electoral test in Toronto. The vote could be held at the same time as another by-election in a Liberal stronghold in Montreal, following the resignation this month of long-time MP Denis Coderre.

Mr. Trudeau's victory in the Liberal leadership race in April was met by a surge in public opinion polls, but the by-elections will help him gauge his support in ridings that the party should easily carry to victory.

At a news conference Wednesday after his last caucus meeting, Mr. Rae specifically nixed any foray into municipal politics in the 2014 Toronto election. He said he is optimistic about the future of his party and knows he is giving up a shot at being a minister in a possible Trudeau government.

"I had a choice to make, and it was a hard choice, and when we make hard choices, we show emotions," he said.

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Mr. Trudeau praised Mr. Rae for taking on the position of interim leader after the party's disastrous showing in the 2011 election. "The history books will also remember someone who was a great leader for the Liberal Party during very difficult times," Mr. Trudeau said. "We will miss his wisdom and his experience, but we will miss mostly his passion."

Mr. Rae became the MP for Toronto Centre after easily winning the seat in a by-election in 2008. He said he has recently been spending much time working with native leaders in Northern Ontario, and plans to dedicate his energies to that cause.

"You may have noticed my occasional absences over the last few weeks. Or worse, you didn't notice my absences," Mr. Rae told reporters. "I will be stepping down as the member of Parliament for Toronto Centre to continue my work as a lawyer, a mediator and someone who gets involved in conflict resolution both in Canada and elsewhere."