In a presidency that has seen almost every institution from the FBI to the Supreme Court consumed by division and controversy, the Federal Reserve is a shining exception. President Donald Trump’s nominees are widely seen as competent, careful and apolitical.

Mr. Trump’s lengthy riff on interest rates Thursday risks tainting that. In an interview with CNBC he declared himself unhappy that Fed Chairman Jerome Powell keeps raising rates. If not an actual violation of Fed independence– he was complaining, not instructing—it was still a jarring departure from the recent tradition of presidents leaving the central bank alone.

He resumed his criticism Friday morning, taking to Twitter to say raising interest rates “hurts all that we have done.” He added, “The U.S. should be allowed to recapture what was lost due to illegal currency manipulation and BAD Trade Deals. Debt coming due & we are raising rates - Really?”

This isn’t just a problem for the Fed, which wants its decisions seen as driven entirely by economics and data; it’s a problem for Mr. Trump. Inserting himself into monetary policy has virtually no upside and plenty of downside.

Start with the fact that on the substance, his complaint is baffling. No president since the 1950s has simultaneously enjoyed such a strong economy and such cheap credit. The expansion is the second longest in history, growth has accelerated to around 3% thanks to massive fiscal stimulus, unemployment at 4% is below most estimates of its natural rate, and inflation sits at the Fed’s 2% target. And yet interest rates, after rising slowly for two years, sit just below 2%—zero in real terms.