Municipal Affairs Minister Linda Jeffrey is set to quit provincial politics to run for Brampton mayor, leaving the Ontario Liberals without a popular incumbent in a crucial battleground ahead of a potential spring election.

Party insiders said Ms. Jeffrey will soon announce her departure from government, setting up a tough fight with scandal-plagued Mayor Susan Fennell for the booming Toronto suburb's top job.

Through her office, Ms. Jeffrey turned down an interview request Sunday.

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A veteran politician known for her calm demeanour, Ms. Jeffrey is a key ally of Premier Kathleen Wynne. An MPP since 2003, she previously served on Brampton council and has long had her eye on the mayoralty, Liberal sources said. She has often been in the spotlight in recent months, leading the government's response to the drug and drink troubles of Toronto Mayor Rob Ford.

Brampton, a suburban city of half a million northwest of Pearson Airport, is part of the so-called 905 belt around Toronto that typically decides provincial elections. Holding seats there, including Ms. Jeffrey's Brampton-Springdale, will be crucial for the Liberals.

Some insiders directly credit Ms. Jeffrey's popularity – and her deep roots in the riding – with keeping the seat out of the Progressive Conservatives' hands in 2011. Other Liberals played down her departure's effect on their electoral chances, saying the party's organization in the area will prevail over an expected tough challenge from the Tories.

Ms. Jeffrey's departure is part of a larger turnover within caucus. She will be the seventh Liberal MPP to leave since 2011. At least four more incumbents are not seeking re-election.

Ms. Wynne controls only a minority of seats in the legislature and must secure the support of at least one other party to pass a budget this spring. If she does not, the province will be plunged into a snap election.

In a preview of the scrappy tone the Brampton mayoral race will take, Ms. Fennell on Sunday blamed Ms. Jeffrey for not bringing all-day, two-way GO train service to the city. As a member of the Liberal government, Ms. Fennell said, Ms. Jeffrey also bears responsibility for the province's $12-billion budget deficit.

"She'll have to stand on her record and her participation in the financial mismanagement of the province," Ms. Fennell said in an interview. "My message to her is: you can't hide in Brampton."

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Ms. Fennell has been recently beset by accusations of lavish spending. In addition to having a taxpayer-funded car and driver, and a vehicle allowance, the Mayor and her staff have expensed hundreds of thousands of dollars in flight upgrades, language classes and other items.

The Mayor said the travel was necessary to court investment for the city and that the $45,000 spent on the car and driver is "good value for money," as it allows her to get work done while travelling between meetings and events.

Despite her troubles, Ms. Fennell is expected to mount a formidable campaign ahead of the October vote. Her re-election bid is being run by Nav Mangat, a seasoned Liberal operative and former provincial political staffer. Mr. Mangat has managed successful campaigns for former federal MP Navdeep Bains in 2004 and MPP Harinder Takhar in 2007; he was also a key player in Mr. Takhar's leadership bid last year.

Ms. Jeffrey will be the second serious challenger to take her on, after Councillor John Sanderson jumped into the race last month.

On top of spending and transparency matters, Brampton faces major development issues, including a push to densify its historic downtown to ease the breakneck growth of McMansion-filled subdivisions on its fringes. Transit is also top of mind, with plans for a light rail line to connect the city with next-door Mississauga.

The married mother of three grown children, Ms. Jeffrey has served in cabinet since 2010, holding the Natural Resources and Labour portfolios before taking over Municipal Affairs and Housing a little over a year ago.

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On the Ford file, Ms. Jeffrey took a careful position: that the province would only intervene if Toronto city council asked it to. Her stance was calculated to strike a middle ground between those demanding Queen's Park step in to stop the madness at Toronto city hall and the desire of the provincial Liberals not to make Mr. Ford into a martyr by being seen to have meddled in city affairs.

Ms. Jeffrey also played an important role in the aftermath of the ice storm that struck Southern Ontario late last year, offering up to $190-million in provincial money to help municipalities repair storm damage.