Michael Bastasch of the Daily Caller documents conservative opposition to a proposed carbon tax.

Seventy-five conservative groups signed a public letter to Congress opposing “any carbon tax” days after reports that Utah Republican Sen. Mitt Romney openly considered backing carbon tax bill.

“We oppose any carbon tax,” conservative groups, led by Americans for Tax Reform, wrote in their letter, which was published online Monday morning.

While the letter is not specifically aimed at Romney, it’s meant to warn Republicans that their conservative base is not in favor of taxing carbon dioxide emissions.

Only a few GOP lawmakers have backed carbon tax legislation, but there’s been a growing lobbying effort by some groups to get Republicans to back a carbon tax as a way to fight global warming.

Big corporations, including oil and gas companies, have increasingly embraced a carbon tax. Exact proposals vary, but supporters often push carbon taxes in exchange for tax cuts elsewhere, fewer regulations or a liability shield against climate change lawsuits.

Romney recently told E&E News he was “looking at” carbon tax legislation put forward by Delaware Democratic Sen. Chris Coons — the same legislation Coons co-sponsored with former Arizona GOP Sen. Jeff Flake in 2018.

“Taxes have never been my intent, but we’ll see what he has to say,” Romney said. “I would very much like to see us reduce our carbon emissions globally, and we’ll see if this might help.”

Romney’s remarks got a strong response from conservative activists opposed to carbon taxes, which they say will hit working-class Americans hardest and do little, if anything, to fight global warming.