How could he do this? Tribe told me via email that it was a simple matter of principle:

I have vigorously defended the legality of President Obama’s executive actions with respect to immigration, health care, and gay rights, all of course without being paid by anyone. Believing that this time he and his administration unfortunately went outside the bounds of the law, I decided to speak up. It wasn’t something I did eagerly because I share the administration’s attitude toward climate change and the urgency of meaningful action on that front, but I’m not willing to keep silent when I see the rule of law threatened just because the threat comes from quarters with which I am in sympathy.

Some people don't buy this explanation, and allege that Tribe has allowed himself to be bought by Peabody Energy, the biggest private-sector coal company in the world. Tribe received an undisclosed fee from Peabody in exchange for writing his comment to the EPA. Dave Roberts of Grist writes that Tribe of "sell[ing] his soul" and making "transparently terrible" arguments. "I don’t know how much Peabody Coal is paying Harvard Law professor Laurence Tribe to serve as its mouthpiece, but it can’t possibly be enough," Roberts writes. Jonathan Chait similarly concludes that Tribe is acting as a mercenary. (EPA's assistant administrator for external affairs, apparently concurring, circulated Chait's column to reporters Wednesday.)

Tribe told me he'd read the proposed rule and made up his mind about its legality before Peabody or anyone else approached him:

Had I turned down the fee Peabody Energy Company offered to have me elaborate my views, I'm sure people would be saying "shame on you for helping that big coal company pro bono." A decision on my part to add to the wealth of a big coal company—a company that hardly needs charity from me—would’ve been just as easy for folks to criticize as it was for them to attack what I did instead, which was to take on this cause on a paying basis, with the express understanding that the views expressed would be entirely my own.

There is a complicated legal debate involved in the matter, wholly apart from Tribe's motivations. Tribe argues that the rule violates the Fifth Amendment because it constitutes a regulatory "taking" by the federal government, limiting a corporation's use of its coal plants without due compensation, and that it violates the Tenth Amendment by coercing states into creating their own CO2 reduction plans or else risking the federal government imposing its own plan. As he notes, that echoes the Supreme Court's arguments in the Obamacare case of King v. Burwell, in which justices wondered whether coercing a state government to follow a federal policy would be unconstitutional.*

Tribe also homes in on a discrepancy between two separate versions of the statutory language, a 1990 amendment to the Clear Air Act, that governs EPA's ability to issue the rule. It's very easy to get bogged down in the details of this disagreement, and in fact the House committee hearing features an impressively detailed argument about the matter, but suffice it to say that there are two separate and contradictory versions of one part of the law, both of which were in the law as it was passed by Congress and signed by President George H.W. Bush. Tribe says one is invalid and the other represents the real law, with the discrepancy being just a clerical error. His opponents counter that the other version is the relevant and statutory one.