Okay, forget all that nonsense about how coal, or nuclear power, or natural gas will decide the Senate’s debate over the climate bill. It really seems to be boiling down to trade.

Today, ten Democratic senators sent President Obama a letter demanding a “level playing field” for U.S. manufacturing in any climate plan. In plain English: If you want your climate bill, you better include “carbon tariffs” to make sure U.S. jobs don’t scurry off to unregulated China.

The list of senators includes some heavyweights, but all are swing votes for the bill—Debbie Stabenow and Carl Levin of Michigan; Robert Byrd of West Virginia; Arlen Specter of Pennsylvania; Evan Bayh of Indiana; and Al Franken of Minnesota. Without the support of these lawmakers, you can stick a fork in the climate bill—it’s done.

“Any climate change legislation must prevent the export of jobs and related greenhouse gas emissions to countries that fail to take actions to combat the threat of global warming comparable to those taken by the United States,” reads the letter.

This whole notion of slapping “dirty” imports with a special tariff, in order to make foreign manufactures adhere to the same environmental standards as the U.S., gained traction (even with Paul Krugman) after the House included mandatory carbon tariffs in the Waxman-Markey bill.