Ryan Avent says those who favor a carbon tax over cap and trade are hurting the climate change bill. Joe Romm makes a more convincing argument along the same lines:

...nothing bugs me more than this notion that Congress would ever pass a “simple” carbon tax, even if it were politically feasible, which it most certainly is not.  Well, one thing bugs me more people who attack the first serious chance we have to get major energy and climate legislation because they are operating under the severe misimpression that the political system of this country might embrace a tax.

Drum seconds Romm:

I don't really understand how it is that smart people don't get this.  Politically, cap-and-trade is the only climate plan that has even a remote chance of getting through Congress, it's the only plan that institutes a firm limit on greenhouse gases, and it's the only plan on the table.  Is it really worth giving all that up for the chimera of a tax that has some esoteric technical advantages on a whiteboard, but in the real world can't pass and wouldn't solve the carbon problem even if it did?  It's hard to see why anyone serious about real-world change would buy into this.

Because we actually believe that a carbon tax will bring green benefits without the kind of crude regulatory scheme that could stimgatize environmentalism for a long time? Because we think it will work better?