Here’s the problem: Tom Coburn cares about the deficit. Grover Norquist doesn’t. People who care about the deficit know you’ll need more revenue to close it. Coburn, who’s trying to negotiate a deal on the deficit and who served on the Fiscal Commission, knows that better than most. His hope is to get the new revenue by closing expenditures rather than raising rates. Norquist opposes that — he opposes anything that produces one more dollar of revenue. And Coburn, after initially trying to accommodate Norquist’s views, has decided to fight. I’ll let Jon Chait take it from here:

Norquist and Coburn have been circling each other for months, trading barbs in the media. Now Coburn is using a test case to expose Norquist’s Pledge. That test case is the ethanol subsidy, which is pork that survives due to the strength of the agriculture lobby, but which the conservative movement at least putatively opposes. The ethanol subsidy, like many subsidies, comes in the form of a tax break. Eliminating it is, therefore, a tax increase. Therefore, eliminating the ethanol subsidy, without using the revenue for a tax cut, would violate the Pledge.

In other words, Coburn has set a trap for Norquist. He has proposed eliminating the ethanol subsidy. If Norquist supports it, he has to alter his pledge to allow for closing loopholes that raise revenue. If he opposes it, he has to admit that he opposes closing loopholes that even Norquist admits are unsupportable. Norquist’s response? He opposes closing the loophole. ... So now the trap is sprung. Coburn can now paint Norquist’s pledge as un-conservative — it’s protecting pork and special interest subsidies that conservatives oppose. And Coburn is right! Assuming, of course, that you define conservative to mean a belief in low nominal tax rates and a tax code that doesn’t pick winners or losers, as opposed to a tax code that raises the smallest amount of revenue as possible from rich people.

Of course, the ultimate question isn’t whether Coburn can score on Norquist, but whether he can convince his colleagues in the House and Senate that he’s right. Violating something like the anti-tax pledge that Republicans sign has to be done en masse. If it’s only one heretic, he or she can be primaried and attacked. If it’s a whole party of heretics, there’s not much anyone can do about it.