(CBS) – When Gov. Bruce Rauner vetoed the statewide school funding bill, he also planted what education advocates are calling a “ticking time bomb” for funding Chicago Public Schools.

CBS 2 Political Reporter Derrick Blakley looks at what that explosive provision could do.

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As he broke ground on $6 million in new athletic fields at both Clemente and Wells high schools, Chicago Mayor Rahm Emanuel again denounced Rauner’s amendatory veto.

“It actually doubles down on the worst education formula in the United States of America,” Emanuel said Thursday.

Rauner made a change that would put big dent in CPS funding down the road. Under the original Senate Bill 1, no school would ever get less money than it received in 2017. But Rauner’s revision wipes out that guarantee, starting in 2020.

What’s more, starting that year, school aid would be handed out on a per-pupil basis, at the very time when CPS enrollment is shrinking.

“Many school districts around the state are going to lose population, but Chicago, as the largest school district, is going to lose more students,” the Civic Federation’s Laurence Msall said.

At CPS, the enrollment trend is already down: in 2012, it was 404,000; in 2016, it was 381,000. This year, it’s an estimated 374,000.

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Fewer students would mean less funding.

“He seeks to punish schools for losing enrollment, a move probably designed to hurt the Chicago Public Schools system, which of course is his favorite target,” state Sen. Andy Manar, D-Bunker Hill, said.

Critics say Rauner’s plan would hurt Chicago another way, too, by failing to compensate for the use of TIFs, which are supposed to spur economic development but deprive schools of millions.

“A school district in Elgin or Chicago or anywhere else that has a tax increment financing district doesn’t have access to that property tax for 23 years,” Msall says.

Manar says: “This pits job creation efforts against public school funding.”

In a statement, a spokesperson for Rauner denies his veto is an attack on Chicago.

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The governor insists his plan would increase CPS funding this year. But both Democrats and independent watchdogs say it would decrease funding for CPS and for public education statewide down the road.