In this March 16, 2011, file photo, steam escapes from Exelon Corp.’s nuclear plant in Byron, Ill.

A coalition of labor groups and a bipartisan handful of state lawmakers want to overhaul the state’s renewable energy plans and keep the state’s nuclear fleet humming, but it’s not clear how much the ambitious change would cost ratepayers.

Climate Jobs Illinois, a nonprofit backed by many of the state’s most prolific public and private unions, announced the Climate Union Jobs Act on Monday morning.

Similar to the Clean Energy Jobs Act, the about-500 page bill would put Illinois on a path to 100 percent renewable energy. It ends the rate formula enacted in the 2011 Energy Infrastructure Modernization Act, provides aid for laid-off workers because of closures and returns to traditional rate-making. It also includes significant investments in green energy programs and creates an alternative wholesale energy market that would be run by the state.

Where it differs is in its concessions to labor. CUJA would put requirements on energy companies to only work with in-state union labor in a number of situations, change project labor agreements and make the organizations promise not to interfere with their employees unionizing.

It would offer power company Exelon’s four nuclear generation facilities — Braidwood, LaSalle, Bryon, and Dresden — 74 million megawatt-hours of “Carbon Mitigation Credits” in an effort to keep them afloat. Those four locations were said to be in imminent danger of closure. Other locations in Exelon’s fleet wouldn’t be eligible because they receive zero-emission credits. Supporters said the move to all-renewable energy can’t happen at once, and nuclear power is the best bridge.

“We do realize that we have to find a cleaner way of generating electricity and to focus ourselves on this climate issue that we are dealing with,” said Rep. Larry Walsh, D-Elwood. “To do that, we can’t just shut the switch off and go to something else.”

Sen. Sue Rezin, R-Morris, is also a sponsor.

“This legislation puts working families at the center of Illinois’ clean energy efforts — where they should be,” she said. “By preserving the Dresden nuclear plant, we can keep delivering hundreds of middle-class jobs for families and carbon-free electricity for the state.

The plan’s ambition needs to be backed with money. That would come from doubling Illinois’ Renewable Portfolio Standard budget, which ultimately comes from ratepayers.

“While we have some costs and investments included in our bill, there are many, many benefits I believe will far outweigh what those costs will be,” said Illinois AFL-CIO Secretary Treasurer Pat Devaney. He didn’t specify how much more monthly bills would cost under the plan.

The bill expands aid to low-income ratepayers but doesn’t appear to offer any assistance to businesses, who often bear the lion’s share of utility bills.

Exelon, which announced it would be splitting its energy generation interests from its utilities, responded to the announcement.

“Exelon has consistently said that Illinois urgently needs a bold and comprehensive energy bill that preserves existing clean energy resources, expands investment in renewables, creates good jobs and ensures the economic and environmental benefits are shared equitably,” said company spokesman Paul Adams. “As we have with other stakeholders who have proposed energy legislation, we’re committed to engaging with CJI on the additional work that is required to preserve nuclear plants and achieve the state’s ambitious environmental goals. We expect that this and the many other proposals in Illinois will be considered together.”

Exelon executives have stressed that inaction likely will result in plant closures.

The dynamics of working with Exelon and their company ComEd have become complicated in the months after the release of a deferred prosecution agreement admitting to a years-long patronage scheme by some of the utility’s executives to curry favor with former House Speaker Michael Madigan, who has denied any wrongdoing.

When asked, officials didn’t say if they had a hand in crafting the legislation, instead saying they consulted with CJI and others during previous working groups.

“They’re not going to write this bill,” said Rep. Jay Hoffman, D-Swansea, referring to Exelon.