Investors faced yet another bumpy start to the trading week with US assets coming under fresh pressure, although it’s mounting concern over American debt rather than tariffs generating the volatility this time.

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

The move came as Republicans seek to approve a large package of tax cuts, spending hikes and safety-net reductions which could add trillions of dollars in U.S. debt.

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.

Moody’s lowered the U.S. credit score to Aa1 from Aaa on Friday, joining Fitch Ratings and S&P Global Ratings in grading the world’s biggest economy below the top, triple-A position. The one-notch cut comes more than a year after Moody’s changed its outlook on the U.S. rating to negative. The credit assessor now has a stable outlook.

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

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