President Donald Trump has opened up a new front in his attack on the Federal Reserve and its chair, Jerome Powell: He says the alledged mismanagement of a building renovation project could be grounds for firing Powell.

The independence of central banks, which allows policymakers to operate free from political meddling, is considered sacrosanct by investors and economists.

Three things can simultaneously be true: that it would be reasonable for the Federal Reserve to cut interest rates; that its headquarters renovation is too expensive; and that the Trump administration's attacks show why central banks are designed to be independent in the first place.