When Harry Reid began attacking the billionaire Koch brothers, plenty of political pros wondered what game the Democratic leader was playing. Now that his strategy is becoming clear, the question is whether Republican donors will let him rob them of their best chance in years to roll back the Obama agenda.

The Senate majority leader since last year has been regularly assailing Charles and David Koch, accusing the libertarian donors of endless misdeeds and trying to "buy the country." The Koch-bashing strategy appears...