Toronto Mayor Chow

NDP MP Olivia Chow's resignation from her Trinity-Spadina riding means the beginning of a hotly contested bylection race to replace her. (Sean Kilpatrick/Canadian Press)

The downtown Toronto riding of Trinity-Spadina was just vacated Wednesday morning, but already one nomination candidate has announced her interest and others are lining up for a federal byelection.

NDP MP Olivia Chow resigned her federal seat Wednesday morning before launching her Toronto mayoralty bid against incumbent Mayor Rob Ford.

Christine Innes, on her website, has already announced her candidacy for the Liberal nomination. This morning she tweeted, "Thanks so much to my friends at the Carpenters' Union for full endorsement of my nomination candidacy."

Innes, who ran against Chow twice before, is currently chief of staff to Ontario Tourism Minister Michael Chan, and was co-chair of the Liberal 2013 leadership convention that elected Justin Trudeau as party leader.

She's married to the former Liberal MP for the riding, Tony Ianno.

Insiders say Innes and Ianno have been working for months in anticipation of Chow's resignation and have signed up thousands of Liberal Party members.

One Liberal said this is "make or break time" for Innes. Another said that for Innes, "Third time's a charm." Yet another said the candidate was "unlikely to be Christine."

Chow had a 20,000-vote margin

The riding was won by Chow in 2011 with a 20,000-vote margin, and overcoming that edge is a formidable task.

Some Liberals say this race will be the true test of Trudeau's ability to attract votes for Liberals, a much tougher challenge than winning the November byelections in Bourassa and Toronto Centre, both already Liberal strongholds.

Chow's timing means a Trinity-Spadina byelection will take place in the riding's current boundaries, encircling downtown Toronto landmarks such as the Eaton Centre and the CN Tower as well as part of the University of Toronto, Kensington Market and Chinatown.

Trinity-Spadina has been an NDP riding since 2006 when it was captured from the Liberals by Chow, a former city councillor.

Several possible contenders for the NDP nomination

For the NDP, one of the strongest candidates, if she runs for the nomination, would be Jennifer Hollett, a former TV host and videographer for MTV and CBC.

Hollett vied for the NDP nomination in Toronto Centre but lost to Toronto writer Linda McQuaig. McQuaig was subsequently defeated in the byelection, losing to current Liberal MP Chrystia Freeland.

A community activist and Harvard graduate, Hollett has been touted as a natural candidate for a new riding, to be known as Spadina-Fort York, that will spring out of Trinity-Spadina after electoral redistribution in 2015.

Asked if she intends to seek the nomination in Trinity-Spadina, Hollett said in an email to CBC News, "I'm honoured to be considered, but it's too soon to make any decisions at this time."

Another possible candidate for the NDP nomination is Joe Cressy, director of the Stephen Lewis Foundation, who presumably might have to rethink any idea of working for Chow's mayoralty race if he were to become a candidate.

Mike Layton, a Toronto city councillor and son of former NDP leader Jack Layton, who died of cancer 2½​ years ago, has been viewed as a potential candidate. His executive assistant, however, confirmed to CBC News that Layton is not interested in pursuing a federal seat.

Joe Pantalone, a former Toronto city councillor popularly known as "Joe Pants," is also being talked about as a potential candidate.

An NDP insider said the party will be announcing a nomination contest date within days.

No potential Conservative candidates have stepped forward yet. In 2011, Conservative Gin Siow managed to garner just over 16 per cent of the vote in Trinity-Spadina.

A date for a byelection for Trinity-Spadina will have to be set within six months by Prime Minister Stephen Harper, after consulting the Governor General.

The race will add to Toronto's politically active scene, likely arriving at some point during a period that will encompass a mayoralty race by fall and a possible provincial election this spring.