Olivia Chow has resigned her seat in Parliament, effective immediately, and is set to launch a bid to become the next mayor of Toronto.

A party spokeswoman confirms the Toronto New Democrat tendered her resignation to the Speaker of the House of Commons earlier today.

The dynamics of the race are expected to change dramatically once Ms. Chow, a former councillor and wife of the late New Democrat leader Jack Layton, enters the fray. She will be the first serious left-wing candidate to challenge scandal-plagued Mayor Rob Ford.

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The other leading candidates – former Progressive Conservative leader John Tory, Councillor Karen Stintz and former city budget chief David Soknacki – are all on the centre-right.

The launch is scheduled for a church in St. James Town, the crowded inner-city neighbourhood of tenement buildings where Ms. Chow's family lived when they first arrived in Canada, one source said.

Ms. Chow is expected to present a starker contrast to Mr. Ford's agenda. The source said she will run as the "change" candidate, promising to steer Toronto away from the track Mr. Ford has put it on in recent years.

In a text message, Councillor Doug Ford said, "The Mayor welcomes Olivia to the race." John Capobianco, Mr. Tory's campaign co-chair, said his candidate is focusing on his own campaign: "We are confident that Toronto taxpayers want a livable, affordable and functional city and John is the only candidate that can make it happen," he wrote in an e-mail.

Instead of simply presenting herself as "Rob Ford without the drama" – as the source characterized the other candidates' campaigns – she will break sharply from Mr. Ford's policy agenda.

She will, however, present herself as fiscally responsible, in an effort to block expected attacks that electing a social democrat will lead to big spending at city hall, sources close to her said.

The sources said she will also make a play for voters who supported Mr. Ford in 2010.

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Her campaign, for instance, will target working-class people in Toronto's suburbs – Scarborough, Etobicoke and North York – who broke in Mr. Ford's favour four years ago. Much of her campaign's message will be based on her personal story as the daughter of immigrants. She will be presented, one source said, as someone who "knows the value of a dollar."

Her political base, however, is mainly downtown and in the inner-city, where she represents the riding of Trinity-Spadina.

The popular Ms. Chow has long been under pressure to enter the fray.

People close to her have been setting up campaign infrastructure for more than a year. In late 2012, when Mr. Ford was nearly booted out of office by a court over a conflict-of-interest matter, Ms. Chow had a campaign ready to run in the possible by-election to replace him.

Since that time, her behind-the-scenes campaign has only grown, attracting the support of such big names as George Smitherman, the former Ontario deputy premier who lost the 2010 mayor's race to Mr. Ford, filmmaker Deepa Mehta and ex-provincial cabinet minister Frances Lankin. Ms. Chow's chief campaign organizers are NDP activist Joe Cressy and John Laschinger, the mastermind of David Miller's 2003 mayoral triumph over Mr. Tory.

Ms. Chow's campaign also includes more than 40 fundraisers and an army of volunteers.

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Born in Hong Kong in 1957, Ms. Chow immigrated to Canada at age 13.

In her time on city council in the 1990s and 2000s, Ms. Chow was known for her social policy work, particularly on homelessness. Since her election to Parliament in 2006, she has often worked on urban issues, including as transportation critic, where she has pushed for more federal government help building public transit and expressways.

The crowded field demonstrates the sheer amount of interest in taking on Mr. Ford, who appears vulnerable after spending most of the last year dealing with scandals, including admitting to smoking crack cocaine and driving drunk.

No other prominent left-wing candidates have entered the fight, however, leaving a clearer path for Ms. Chow.

With a report from Gloria Galloway and Canadian Press