That legislation, which the Congressional Budget Office estimated will add $4.5 trillion to the deficit through 2034, proved to be the final straw for Moody’s. The credit agency had previously kept the U.S. in its top rung of borrowers since it first issued ratings for government bonds in 1919.

Investors sold stocks and bonds after Moody’s downgraded the U.S. credit rating, potentially complicating negotiations around Republicans’ tax plan.

Investors sold U.S. government bonds and the dollar on Monday amid concerns about the U.S. fiscal picture. Stocks edged higher.

Moody’s gave the U.S. a negative outlook—but markets didn’t flinch. This isn’t about default. It’s about trust, and why credibility erosion is the real risk.

After the United States lost its last perfect credit rating on Friday, Republicans and Democrats responded by pointing fingers at each other.