For years, Illinois lawmakers and governors have turned to the state's road fund in times of financial distress, repeatedly using money drivers cough up at the pump and secretary of state's office to patch budget holes instead of repaving highways or repairing railways.

But that money pot soon could be declared off limits. Voters will see a question on the Nov. 8 ballot asking if the Illinois Constitution should be changed to prevent the state from raiding funds intended to be used on transportation projects.

If the so-called Safe Roads Amendment is approved, all money collected through gas taxes, tolls, driver's license fees and vehicle registration stickers would be put into what amounts to a budget "lockbox." That means those dollars could only be used for transportation-related expenses, such as road construction, enforcing traffic laws, mass transit and expanding airports.

What's behind this out-of-nowhere push to preserve the road funds? As is usually the case in politics, just follow the money.

The referendum is heavily backed by those who would most directly benefit — asphalt companies, contractors, engineers, excavators, carpenters and labor unions whose members lift the shovels and lay the pavement. As of early October, Citizens to Protect Transportation Funding has raised $3.3 million to build support for the ballot initiative among the public, including a $1 million television ad buy.

Among the largest contributions is $1 million from the Fight Back Fund, headed by Marc Poulos, executive director of the Indiana, Illinois, Iowa Foundation for Fair Contracting, which represents the powerful International Union of Operating Engineers Local 150. The fund is classified as a tax-exempt political advocacy group, meaning it does not have to disclose its donors, though Poulos said it's funded largely by union members.

That's led to some criticism, mostly from newspaper editorial boards, that the proposed change amounts to a windfall for unions. Meanwhile, other groups like the Civic Federation, a nonpartisan budget watchdog, argue the amendment would eliminate the flexibility the state may need to tap into special funds given the ongoing budget crisis.

Those arguments come as Republican Gov. Bruce Rauner has spent much of his tenure attempting to limit the power of unions in Illinois, which he says have too much influence over the legislature and state government. Democrats, meanwhile, have only strengthened their resolve to protect their traditional political allies in organized labor, contending Rauner's attacks on unions would undermine the middle class.

If Rauner has objections to the constitutional amendment, he's not sharing them publicly. Asked whether he had concerns the measure would tie his hands or amount to a jackpot for unions, Rauner said, "There's a lot of pluses and minuses to it."

"At this point, it's in front of the voters," Rauner said. "Let's let the voters decide."

Indeed, Rauner had no say in whether the question ended up on the ballot. That was up to lawmakers, who during the spring session voted overwhelmingly to put the amendment question on the fall ballot. Just four legislators voted against the proposal.

The measure's popularity at the Capitol is a demonstration of what's at stake politically. Lawmakers like to cut ribbons on new projects back home, but with less money to go around amid the state's historic budget impasse, opportunities for good publicity are few and far between.

While transportation advocates have called for a tax hike to pay for transportation projects, that's a tough sell at any time, but especially during an election year. Protecting those dollars through a constitutional amendment was a more palatable option. Anti-tax Republicans would benefit from the influx of transit money that drivers already are forking over, and Democrats can keep unions happy as they ask for labor's help on the campaign trail as Rauner pours millions of dollars of his own money into House and Senate contests as he seeks to diminish Democratic control of state government.

Furthermore, advocates like the Metropolitan Planning Council note the change could help ease the way for an eventual tax increase to modernize the state's aging infrastructure, saying people can't be asked to pay more if there's no guarantee the money will go to upgrading roads and bridges.

It's not just road builders who are backing the amendment. Dozens of organizations have lined up behind the proposal, uniting conservative groups with agricultural interests, the business community and unions. Their reasons are varied.

The anti-tax group Americans for Prosperity argues the change will force more transparency and honesty when it comes to budget-making, saying taxpayers should know where their money is going. Unions want to put members to work. Businesses want to make it easier to distribute and transport products, and to make sure their workers can report for duty on time.

"There are an awful lot of arguments in favor," said Todd Maisch, president of the Illinois Chamber of Commerce. "For us, it really starts at the very obvious notion of how important our transportation infrastructure is to the Illinois economy.

"When you think of the importance of agriculture to the Illinois economy, if you can't get agriculture products from the field to the elevator or the processing plant because of the inability to put fully loaded trucks on certain rural stretches, that's a very real and immediate concern."

Instead of paying for services out of one pot of money, state government relies on a complex system that includes hundreds of specialized funds. In theory, fees and taxes are paid by users of a specific service, which in turn are used to operate related programs. For instance, fishers buy special licenses, and the money collected is then used to stock or maintain waterways.

But with state government facing a severe cash shortage, those special funds are often inviting targets for lawmakers and governors looking to keep universities open, pay for programs for the elderly and provide food for prisoners.

That's particularly true for the Road Fund, which is one of the largest and most flush funds because every driver pays into it through gas taxes, registration and other license fees. And since that money usually isn't spent until the summer construction months, it has all winter to swell again — making it seem like an easy source of money when lawmakers and the governor turn to crafting a budget each spring.

Last year, Rauner and lawmakers agreed to drain $500 million from various transportation-related funds as they sought to plug a $1.6 billion budget hole. Of that, $250 million came from the state's Road Fund. Another $50 million came from a fund earmarked for construction.