Several free market and conservative groups called Democratic senators tyrants Tuesday for criticizing their opposition to global warming policies on the Senate floor.

The Heritage Foundation, Heartland Institute, American Legislative Exchange Council, and Americans For Tax Reform, among others, lashed out Tuesday in a letter obtained by The Daily Caller News Foundation at Democratic senators Harry Reid, Barbara Boxer, Al Franken, and Sheldon Whitehouse for calling them conspirators in a “web of denial.”

“We hear you,” the letter read. “Your threat is clear: There is a heavy and inconvenient cost to disagreeing with you. Calls for debate will be met with political retribution. That’s called tyranny. And, we reject it.”

The 22 groups who signed the letter have various policy views, including on global warming. They argue in the letter the senators’ antics violated the groups’ rights to free speech, as well as their freedom of association, and that the senators’ knocked U.S. democracy off balance.

“The delicate balance of our democracy is preserved when all groups are free to speak in the public square, and ALL Americans should be concerned when agents of the government use their official offices to marginalize political foes,” the letter reads.

“You were elected by the people to build consensus and find compromise — to fiercely debate the most pressing issues of our day. Often, these debates are meant to be contentious and without a clear solution because innovation comes from great challenge,” the letter adds. “Sadly, our democracy and our freedom hangs in the balance as you use your office to bully and single out groups to blame rather than ideas to debate.”

Whitehouse, the Democratic senator spearheading the hit parade, directed 19 of his fellow Democratic senators to attack some of the conservative and libertarian groups included in the letter on the chamber floors for engaging in what the senators call a “web of denial.”

Whitehouse has a history of tormenting organizations for refusing to break free from oil producers such as Exxon Mobil.

The American Geophysical Union (AGU), which boasts more than 62,000 members worldwide, announced in May it would review and possibly reconsider a decision it made in April to continue its relationship with Exxon after Whitehouse and California Rep. Ted Lieu, a Democrat, pressured the group to divorce from the company.

Whitehouse and Reid — according to an internal email circulated by Emily Enderle, a top environmental policy adviser to the Rhode Island senator — created a senate schedule for Monday and Tuesday, requesting the senators lambaste the groups assigned to them.

Enderle coordinated her efforts with several leading environmental groups, she explained in the email — some of the groups include the Sierra Club, Union of Concerned Scientists, and the Clean Water Action.

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