Improper purpose: That concept lies at the heart of the striking opinion by Chief Judge James Boasberg of the federal district court in Washington, which quashed the Trump administration’s subpoenas aimed at Federal Reserve Chair Jerome Powell.

Rather than focusing on procedural irregularities, which lawyers love to do, the judge homed in on the far more obvious point: The criminal investigation of Powell was really intended to pressure him to lower interest rates. That made the subpoenas improper because Fed independence is protected by law. And it is a general principle of constitutional law that the government may not use indirect pressure to do what it may not do directly.

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