A potentially major new feud over climate change has drawn the attention of Congress. Earlier this year, 17 Democratic state attorneys general began issuing subpoenas to Exxon Mobil over whether the oil giant hid its research on climate change. The company adamantly denies a cover up.

Federal lawmakers have waded in. Congressional Republicans argue the investigations violate Exxon’s right to free speech and other constitutional protections. Democrats defend the attorneys general, and are urging the Justice Department to undertake its own investigation.

The attorneys general were motivated by media reports that Exxon scientists knew about the climate change risk for decades but the company decided to withhold the data. Democrats contend the GOP's free-speech argument "reprises the tobacco lawsuit's own early history of efforts from Congress to discourage or interfere" with the 1990s legal settlement under which tobacco companies agreed to pay states annually for public health damage caused by cigarette smoking.

"The people following Exxon are trying to follow the precedent of tobacco litigation," said Brendan Collins, partner with the law firm Ballard Spahr and an expert on environmental regulations. He said "the reality is a very different situation" because the evidence brought against the tobacco industry two decades ago is "pretty substantially different from the idea that Exxon may have duped me from getting a low-mileage [car] and now the island of Tuvalu is going to get covered by water."

In contrast to the effect of tobacco smoking on human health, it's a "very big leap" to link what Exxon "did or didn't do" to the harm posed by climate change, Collins said.