"I refuse to stand on the sideline while we can deliver change," Chow said Tuesday, pointing to the NDP's plan to provide one million affordable daycare spaces across the country.
"Tom Mulcair is the only leader that can defeat Stephen Harper," she said.
Chow is running for the NDP in the upcoming federal election in the same general downtown Toronto area she represented for years, although her former riding now has a new name and different boundaries.
Chow, the widow of onetime NDP leader Jack Layton, resigned her seat in Parliament in 2014 to run in Toronto's mayoral election, which she lost to John Tory. Now, she'll face off with former city councillor Adam Vaughan, who captured the Trinity–Spadina riding for the Liberal Party last summer.
That riding, which Chow represented for eight years, was redistributed and renamed Spadina—Fort York in the redrawing of Canada's electoral boundaries last year that will see the addition of 30 ridings in this election.
Spadina—Fort York encompasses much of the downtown core as well as the city's waterfront, a space that's seen a tremendous condo boom.
Tough race against Vaughan
Layton's children, Sarah and Mike Layton, appeared with Chow and Mulcair at Tuesday's press conference.
Despite her popularity, she's likely to face a tough race against Vaughan, a former journalist, who is also popular in the downtown riding.
Chow emphasized that she had already represented many parts of the riding, as well as her work in Toronto on affordable child care. Asked about how she would take on Vaughan, Chow spoke of past Liberal Party promises to provide a national daycare program.
"Nothing personal against Adam Vaughan. It's the Liberal Party that disappoints," she said.
"I remember the disappointment I felt in '93 at the Red Book promise for a national child--care program. I remember '97, disappointed again. And then there was 2000. It was over and over again. After 13 years of Liberal government, they just didn't get it done."
Chow, also a former Toronto councillor and school trustee, is currently serving as a distinguished visiting professor with Ryerson University's Faculty of Arts, where her work focuses on community engagement and democratic participation. Chow said she'll take a leave of absence from Ryerson.
In the 2014 mayoral election, Chow won 226,879 votes — or 23 per cent of the overall vote — to finish third behind Tory and Doug Ford, the brother of former mayor Rob Ford.
In the 2011 federal race, Chow won the previous riding by more than 20,000 votes.