President Donald Trump lashed out at the Federal Reserve today after administration officials spent the weekend trying to assure the public and financial markets that Jerome Powell’s job as Fed chairman was safe.

“The only problem our economy has is the Fed,” the president tweeted today. “They don’t have a feel for the Market, they don’t understand necessary Trade Wars or Strong Dollars or even Democrat Shutdowns over Borders. The Fed is like a powerful golfer who can’t score because he has no touch — he can’t putt!”

On Wall Street, stocks had already been down but intensified their fall after Trump’s tweet. Markets were already facing their worst month in a decade over fears about a U.S. trade war with China, a slowing global economy and chaos in the Trump White House. Near the close of a holiday-shortened trading session today, the Dow Jones Industrial Average was off more than 500 points.

Trump’s latest tweet attacking the Fed was met with concern that any attempt to diminish Powell or remove him as chairman could destabilize the economy.

“He is seeking open warfare on Christmas Eve,” said Peter Conti-Brown, a financial historian at the Wharton School of the University of Pennsylvania. “We’ve never seen anything like this full-blown and full-frontal assault. This is a disaster for the Fed, a disaster for the president and a disaster for the economy.”

The president has expressed frustration over the Fed’s decision to raise its key short-term rate four times this year. Those moves are intended to prevent the economy from overheating at a time of brisk growth and an unemployment rate near a half-century low.

At a news conference last week, Powell explained that the rate hikes were evidence of the economy’s strength. But Trump sees the increases — which lead to higher borrowing costs for consumers and businesses — as an economic and political threat.

Trump’s attacks are widely seen as an intrusion on the political independence of the Fed, which exists to determine the flow of money based off economic data on employment and inflation.

Fed independence has long been among the bedrocks of the U.S. financial markets. It ensures that central bankers can make politically unpopular decisions, such as fighting high inflation in the 1980s or rescuing banks after the 2008 financial crisis.

Sen. Richard Shelby of Alabama, a senior Republican on the Banking Committee, has cautioned against removing Powell.

“I’d be very careful doing that,” Shelby told reporters Saturday at the Capitol. “The Federal Reserve is set up to be independent.”

Stocks did decline after Powell announced this year’s fourth rate hike on Wednesday. But the sell-off appeared to reflect concerns that the Fed was moving too fast in its plans to raise rates and shrink its vast portfolio of bonds given an economic slowdown that is expected in 2019.

Fed officials voted unanimously to increase rates last week. Among those voting with Powell were three other board members who were chosen by Trump: Richard Clarida, Randal Quarles and Michelle Bowman.

The president expressed his displeasure today with the Fed after Treasury Secretary Steve Mnuchin had tweeted on Saturday that Powell’s job was safe.

Mnuchin also tweeted on Sunday that he had checked with the heads of the six largest U.S. banks to ensure that they had enough liquidity to operate in a stock market that has tumbled sharply since October.

“My sense is the Mnuchin tweets don’t tell us much about the economy, but they provide unusual insight into the chaos inside the White House,” said Joseph Brusuelas, chief economist at the consultancy RSM. “Rather than instilling confidence, it created confusion and raised more questions than it answered. Foremost among those is, how safe is the job of Jay Powell as chairman of the Federal Reserve?”