Bills in Pennsylvania and Ohio to prop up troubled nuclear-power plants face growing opposition from businesses and advocacy groups, who say the plans are too costly and would unfairly tip the scales in energy markets.

Four nuclear plants are slated for early retirement in the two states. Exelon Corp. has warned that it would close its Three Mile Island Generating Station in Pennsylvania in September if it doesn’t get government aid. The money-losing facility, along the Susquehanna River near the state capital of Harrisburg, became a symbol of the risks of nuclear power when it suffered a partial meltdown in 1979.

Many locals don’t want to see it close. “We’re totally sold that we have to help bail out this nuclear industry,” said Stephen Mohr, a longtime supervisor in Conoy Township, which sits 300 yards from one of the plant’s cooling towers. “We can’t afford to go without any one of them,” he added, referring to nuclear-power plants.

In Ohio, Gov. Mike DeWine said he supports saving the state’s two nuclear plants to keep carbon emissions down. The plants in northern Ohio produce 90% of the state’s zero-carbon electricity, according to industry and state officials, and employ 1,400 people. One proposal would charge residential customers $2.50 a month to create a $300 million fund, partly for propping up the struggling facilities.

“I’m all for wind, I’m all for solar,” Mr. DeWine, a Republican, said earlier this month. “But you cannot hit the numbers without using nuclear.”