The Fitch Ratings credit-rating agency has downgraded the U.S. government’s long-term debt from AAA status to the lesser AA+ rating. The move could make it harder for the Treasury to attract investors in U.S. government bonds. On Wednesday, U.S. Treasury Secretary Janet Yellen blasted the decision as “entirely unwarranted.”

Treasury Secretary Janet Yellen: “At the end of the day, Fitch’s decision does not change what all of us already know, that Treasury’s securities remain the world’s preeminent safe and liquid asset and that the American economy is fundamentally strong.”

In downgrading the U.S. credit rating, Fitch analysts cited a “steady deterioration in standards of governance over the last 20 years.” This follows drawn-out negotiations between the White House and Republican leaders over the debt ceiling, which narrowly averted a default on the nation’s debt in June.