Toronto-Centre MPP Glen Murray broke the news in a Twitter message on Saturday that he is stepping down as the province's minister of training, colleges and universities — a necessary step for those who want to enter the leadership race to succeed Premier Dalton McGuinty as party leader.
Murray called it a "difficult decision" and said he would release more details in a "special announcement" Sunday.
A spokeswoman would not discuss Murray's plans, saying those would be laid out at the announcement, which is to be held at Toronto's Maple Leaf Gardens — the same site earmarked for the party's leadership convention on Jan. 25.
Murray's Twitter revelation was quickly followed by one from McGuinty, who announced Community and Social Services Minister John Milloy will look after the post-secondary portfolio on top of his current duties. The change is to take effect in a few days.
Murray, who served as the mayor of Winnipeg from 1998 to 2004, was first elected to the Ontario legislature in a 2010 byelection.
His win maintained the Liberals' grip on the downtown riding, which has been a stronghold for the party since George Smitherman was first elected in 1999. Smitherman left the post to enter the Toronto mayoral race, which he lost to Rob Ford.
In 2011, Murray was re-elected and appointed to cabinet in the Liberals' minority government.
In the weeks since McGuinty's surprise resignation on Oct. 15, several ministers have expressed interest in his job.
McGuinty served nine years as premier and 16 years as leader of the Ontario Liberals.
On Friday, he confirmed that former Municipal Affairs and Aboriginal Affairs Minister Kathleen Wynne was throwing her hat in the ring after she stepped down from cabinet, but an official announcement was not expected before Monday.
Wynne, 59, was first elected in 2003 and has also been in charge of the high-profile education and transportation portfolios.
Other potential candidates said to be considering a bid include former cabinet ministers Sandra Pupatello and Gerard Kennedy.
Kennedy was reportedly holding a meeting in Toronto Saturday to gauge whether there was enough support if he decided to join the race.
In 1996, the former Ontario education minister lost to McGuinty by just 140 votes.
Meanwhile, some high-profile members in McGuinty's caucus have already rejected the idea of running.
Finance Minister Dwight Duncan, Energy Minister Chris Bentley, Education Minister Laurel Broten and Economic Development Minister Brad Duguid have all decided to stay out of the race.
Those interested in the job must submit their names to the party by Nov. 23.
The fee to enter is $50,000 and candidates can spend up to $500,000 on their campaign.