The Environmental Protection Agency hasn't made available key data used to craft its twin proposals to reduce carbon emissions from new and existing power plants, 13 Republican attorneys general said.

The attorneys general charge that the EPA is violating federal law by not disclosing the information, which they said makes it difficult for states and the public to ascertain how the agency developed its proposals.

"Without such missing data and related materials, states and the public cannot properly determine the basis on which EPA claims that these emissions standards are achievable and reasonable," said the attorneys general, led by West Virginia's Patrick Morrisey, in a Monday letter to EPA Administrator Gina McCarthy.

Republicans in Congress and governors' mansions across the country have sought to stop, or at least weaken, the proposed rules. They, with allies in the business community, say the regulations would raise energy prices and restrain economic activity.

One of the EPA's proposed rules would essentially block construction of new coal-fired power plants that lack carbon capture and storage technology, which the industry says is too expensive to do feasibly. The other, more far-reaching one would hit existing power plants, as it seeks to cut the power sector's emissions 30 percent below 2005 levels by 2030.

EPA spokeswoman Liz Purchia said she was confident the agency's proposed rules were on sound legal footing.

"[H]istory has shown that EPA writes solid rules and they stand up in court β€” courts have reaffirmed our science and reasoning time and time again. The Supreme Court made clear in 2007 and recently reaffirmed that EPA has an obligation to limit carbon pollution because it’s a harm to human health," she said Tuesday in an email to the Washington Examiner.

The rules' proponents say cutting carbon missions would reduce the effects of climate change, averting costs down the road from rising sea levels and reduced agricultural productivity. They also would reduce medical expenses by limiting sulfur dioxide and nitrogen oxide emissions that would come with older plants coming offline, as that would reduce heart and respiratory ailments, supporters contend.

An overwhelming scientific consensus says that humans are driving climate change, largely by burning greenhouse gas-emitting fossil fuels that warm the planet.

But the attorneys general say the EPA needs to do a better job of showing how it concluded states could make adjustments in the four "building blocks" of their power systems β€” increasing efficiency of coal-fired units, using latent natural gas capacity, developing renewable energy and implementing demand-cutting energy-efficiency measures β€” to reasonably achieve the cuts the agency would require.

For the existing power plant proposal, the attorneys general said missing data made it difficult to figure out how the EPA determined power plants could be fine-tuned to increase efficiency by up to 6 percent. That is a key element for how coal-heavy states are to meet their emissions targets by 2030, but some industry experts say the 6-percent efficiency improvement is ambitious.

"This information is critical to assessing EPA's claims that states and industry will be able to comply with the four 'building blocks' in the proposed" existing power plant rule, which is due for finalization next June, the letter said.

The letter was signed by attorneys general from Nebraska, West Virginia, Oklahoma, South Dakota, Wyoming, Ohio, Alabama, Indiana, South Carolina, Kansas, Louisiana, Montana and North Dakota.