Around $110 million has been spent by both sides on the amendment.
Ads flood airwaves as IL billionaires battle over proposed graduated income tax amendment
"This level of spending is unprecedented, and what's interesting is the two people duking it out have spent more than $50 million each," said David Greisng, president and CEO of the Better Government Association.
A billionaire governor versus the richest person in Illinois - Governor JB Pritzker and Ken Griffin - both have accounted for about 90% off the spending on the ballot question that affects all Illinois taxpayers.
NEWSVIEWS: Graduated Tax Amendment
"This is no doubt a very important question for all of us who pay taxes, but really only a small number of voices are being heard when it comes to donations for and against that," said Isaac Wink of Reform for Illinois.
The money has gone towards ads, with Pritzker rebranding it the Fair Tax Amendment, and those against calling it the Tax Hike Amendment.
Battle over proposed IL graduated income tax continues as both side blame other for deception
"Is it fair that everyone pays the same rate, or do we trust politicians to be able to micromanage tax rates across the spectrum of income, and that is the fight right now," Greising said.
Right now, Illinois is one of only nine states where everyone pays the same tax rate regardless of income. If he wins, Governor Pritzker is proposing to raise taxes on top income earners from the current 4.95% flat to 7.75% and above.
As graduated tax amendment vote approaches, some Republicans accuse governor of intimidation
Polls show the ballot initiative is a toss-up.
If the graduated tax amendment fails, Pritzker can try again at a future election, but experts say he may get more support if next time he ties it to pension reform.