For years, we’ve been hearing that the combination of childish governance and chronic budget deficits would eventually catch up with the United States, but the US always outran the worrywarts. That era, one in which the Treasury could borrow limitlessly and at low cost, may be ending.

“For the first time in my professional life, we’re seeing a shift, with investors looking askance at Treasury debt,” John Velis, a money manager at BNY Mellon told Politico. The spark looks to have been Moody’s downgrade of the Treasury’s credit rating, but there was no new information in the rating agency’s decision. It did, however, as Velis noted, “focus minds.”

Moody’s is one of three big ratings agencies. Their main business is assessing the risk of default — that the debtor, whether a company or a government, may miss an interest payment or repayment of principal on their loan — and their major way of communicating that risk is a letter rating, looking much like a grade. Each has a different system, but they range from AAA down to C or D (as in default). Some institutional investors are, under government regulation or internal policy, required to buy only high-rated debt. Moody, using its own idiosyncratic system, cut the United States from Aaa, the highest, to Aa1, second highest.