Bob Murray , founder and president of Murray Energy Corporation, in his own letter to Caterpillar earlier this year, chided the company for allying with environmental groups that "have been attempting to terminate the use of coal for decades."

In addition to The National Center for Public Policy Research, think tanks and policy organizations that have signed the letter include: the American Conservative Union, Freedom Works, the Competitive Enterprise Institute, Tennessee Center for Policy Research, Frontiers of Freedom, Illinois Policy Institute, 60 Plus (an association of seniors) and the National Tax Limitation Committee.

The letter cites a recent Congressional Budget Office (CBO) report released in April that found that the oil, gas and coal industries would be particularly harmed by cap-and-trade legislation.

"A cap designed to reduce emissions by 23 percent would result in a 54percent devaluation of coal stock value and a 40 percent decline in coal production," notes the letter, quoting from the CBO report.

The CBO report also found that the poor would be disproportionately harmed by a cap and trade system, indicating that a cap designed to reduce emissions by just 15% would cost the poorest fifth of Americans nearly double what it would cost the wealthiest fifth of Americans, as a percentage of wages, in added energy costs.