Cook County Board President Toni Preckwinkle is telling supporters that she plans to announce the formation of an “exploratory committee” to run for Chicago mayor, WBEZ has learned.

Sources close to Preckwinkle say she has not yet decided if she will in fact become a candidate to succeed Mayor Rahm Emanuel – who announced Tuesday that he will not seek a third term in the February municipal election.

There is nothing formally called an exploratory committee in Illinois politics, though some politicians say they have formed one while they test the waters for making a run.

The sources say Preckwinkle, 71, is getting encouragement to run from some of her biggest financial backers in her long career in city and state politics.

Campaign-finance disclosure reports show her biggest contributors since she first ran for county board president in 2010 include the powerful Service Employees International Union, other labor groups and North Side businessman Fred Eychaner.

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Preckwinkle’s top political aide, Scott Kastrup, did not return repeated calls seeking comment.

Preckwinkle recently became head of the Cook County Democratic Party, succeeding former county Assessor Joseph Berrios.

But she also has deep ties to the party’s left-leaning “progressive” wing, and some former colleagues on the City Council said privately that their ward organizations are poised to circulate petitions for a Preckwinkle mayoral campaign.

The sources said they hope a Preckwinkle campaign could unite a broad, multi-racial coalition and bring order to the chaotic situation since Emanuel’s shock announcement.

Politicians who’ve said this week they’re mulling a run include Democratic congressmen Luis Gutierrez and Mike Quigley, former city school board President Gery Chico and county Clerk David Orr.

Roughly a dozen others had previously said they would challenge Emanuel in 2019 and plan to stay in the race now.

Preckwinkle was alderman of the diverse 4th Ward, which included representing much of Hyde Park and Kenwood, before unseating Todd Stroger as county board president. She was an early supporter of 4th Ward constituent Barack Obama’s political career.

After she was elected county board president, she repealed the unpopular sales tax instituted under her predecessor - only to bring it back in her second term, to help with the county’s long-running financial problems.

But she caught the most criticism for her penny-an-ounce pop tax. The county board repealed that tax on sweetened beverages last year, just months after it had gone into effect - and over the wishes of Preckwinkle, who still insists the county needed the revenue.

The repeal prompted cuts in the county payroll, but she easily won the primary in March against perennial candidate Bob Fioretti and has no Republican opponent in November.

Dan Mihalopoulos is an investigative reporter for WBEZ. Follow him at @dmihalopoulos.