State revenue for April exceeded projections by $1.5 billion, according to a letter sent to legislative leaders by state finance officials.
The windfall all but covers the deficit expected in Gov. J.B. Pritzker's budget.
"Gov. Pritzker remains committed to a financially responsible budget that addresses Illinois' outstanding obligations, and recommends that these additional revenues can be dedicated to the state's statutory (fiscal year 2020) pension payment," said the letter co-authored by Illinois Department of Revenue Secretary David Harris and Office of Management and Budget Director Alexis Sturm.
Most of the additional revenue came from income tax payments.
"More than $4.1 billion in individual and corporate income tax revenues were deposited into the general funds in the month of April 2019, up $1.14 billion or 38% from April 2018 income tax deposits of $3 billion," the pair wrote. "This is also more than $1.5 billion more than internally projected for April 2019."
Using the money for pension payments would allow the state to maintain the current pension payment schedule instead of pushing it off another year.
Barrington Hills Republican state Rep. David McSweeney argued the money should be used to pay down the state's current $6 billion backlog of bills.
"We need to cut spending in Springfield," McSweeney said.
The finance officials offered several explanations for the influx of cash last month.
Those include "the performance of the stock market, better federal reimbursement for Medicaid, the elimination of the federal state and local tax deduction and additional changes in the federal tax law that meant many taxpayers didn't withhold sufficient taxes through payroll deductions, backloading their end-of-year tax payments," Harris and Sturm wrote.
Other states had similar increases, they wrote.
In light of the revenue spike, projections for the following fiscal year's income taxes are being increased by $800 million, as well.
However, some of the reasons could be one-time-only, such as the stock market's performance and taxpayers' adjustments in their withholdings because of the new federal tax law, the letter says.