Gov. Pat Quinn vetoed legislation Sunday that would have increased utility bills and helped Commonwealth Edison improve its electrical grid.

But the bill passed the General Assembly by healthy margins, and legislators could override the veto this month.

Quinn, a Democrat, cited practicality and precedent as he slammed the black ink of his veto stamp onto the bill at a news conference in Chicago. The governor said families and businesses can't afford a rate increase, and he expressed concern about a "very disturbing process" in which ComEd sought relief in the legislature after a disagreement with its regulator, the Illinois Commerce Commission.

Improving the grid is important, Quinn said, but legislated rate increases are the wrong way to do it.

"We cannot allow big utilities to take over and run roughshod over families and businesses," Quinn said before stamping the bill with so much gusto that he sent a pen on the table tumbling to the floor. "We're not going to let the utilities run Illinois."

ComEd argues that the bill is needed to support its Smart Grid program, a modernization plan that it says would create jobs, reduce the likelihood of outages and give consumers more say over their energy consumption. The utility issued a statement Sunday expressing disappointment with the veto and asking lawmakers to pursue an override. ComEd has said that the average residential customer bill of $82 per month would increase by about 40 cents in 2014 and by about 80 cents in 2017 if the bill were enacted.

Quinn, who faces re-election next year, said utility-price stability is key in attracting businesses to Illinois. The governor helped create the Citizens Utility Board in the 1980s, a consumer advocacy group that vouches for affordable rates.

State Rep. Lou Lang, the Skokie Democrat who sponsored the bill in the House, said "the likelihood of an override is strong." Lang expects the override to be complete by the end of May.

Three-fifths of legislators in each chamber of the legislature must agree in order to override the veto and enact the law over Quinn's objections. The original bill passed with more than 70 percent support in each chamber.

This bill is the most recent chapter in an ongoing spat between ComEd and the Commerce Commission. The two sides disagreed about how a previous law on grid modernization should be applied. The dispute spawned a lawsuit and slowed efforts to improve the grid.

Lang said the Illinois Commerce Commission "misinterpreted" the original act, and that this new legislation is more of a "cleanup bill" that gives precise direction about how to set utility rates.

Quinn, who in his veto letter cited "unprecedented legislative interference," maintains that the General Assembly is overstepping and injecting itself into an issue best handled by industry experts on the commission.

"(If) they vote to override," the governor said Sunday, "they're voting to raise rates on the people of Illinois."

mitsmith@tribune.com