With a sweeping overhaul of the tax code on the horizon, two Senate Democrats believe this is the moment to broach the third rail of climate change policy: a carbon tax. The plan by the senators, Sheldon Whitehouse of Rhode Island and Brian Schatz of Hawaii, to level a $49 per metric ton fee on greenhouse gas emissions is widely acknowledged as a long shot. But the lawmakers, along with climate activists and a cadre of conservative supporters, insist the tax reform is a way to create bipartisan support. The senators propose to use a portion of the estimated $2.1...