Nineteen House Democrats have joined dozens of Republicans in pressing the transportation conference committee to include an amendment to prevent U.S. EPA from regulating coal ash as a hazardous substance.

The lawmakers, including liberal and conservative Democrats, want the transportation package to incorporate Rep. David McKinley's (R-W.Va.) House-passed H.R. 2273. It would create a new system for regulating ash dumps, giving states the bulk of the oversight responsibility.

Boosters are rebutting critics in Congress and the environmental community who say the measure has no place in the discussions. The supporters note that coal ash is often used as a cement substitute, reducing the amount of material needed in concrete. Labeling it hazardous, they say, would increase road construction costs.

"Removing it from the mix would drive up the cost of concrete and lessen the availability of funds for upgrading our roads, bridges and other infrastructure," the letter said.

Environmental groups want lawmakers to let EPA decide how to regulate coal ash and call arguments that a hazardous designation would hurt recycling a red herring.

"EPA has not proposed to regulate such beneficial reuse," Rep. Henry Waxman (D-Calif.) said at the conference committee's first meeting Tuesday, calling industry allegations "scare tactics."

In a letter of their own, several environmental groups, including Earthjustice, the Sierra Club and the League of Conservation Voters, urged conferees to scrap McKinley's proposal.

"This amendment leaves communities at risk by perpetuating inadequate state protections that have led to major catastrophes like the [Tennessee Valley Authority] coal ash spill in 2008 and nearly 200 cases of water contamination," they wrote.

"Further," they added, "it removes the authority for the EPA to ever revisit a coal ash-specific federal standard, an unprecedented revocation of EPA's authority to protect American citizens from toxic waste."

Power plants and coal ash recycling companies say they can back up worries about what could happen if EPA is left to its own devices, citing concerns from other government agencies like the Army Corps of Engineers and the Interior Department about their own construction programs.

The lawmakers wrote, "The U.S. Department of the Interior has agreed with industry leaders who have stated that if coal ash is designated a hazardous waste it will no longer be used in concrete."

The letters are part of the lobbying effort ramping up as lawmakers begin hammering out a deal on transportation spending with potential add-ons like the coal ash measure and approval for the Keystone XL oil pipeline.

But such proposals may face an uphill battle, with many lawmakers saying they are unwilling to let them stand in the way of progress on the broader package.

Click here to read the letter from McKinley and other lawmakers.

Click here to read the letter by environmental groups.