Donald Trump has made a lot of enemies during his short political career, but it’s one rift in particular, which has many investors nervous. On September 26, Trump launched an attack on the Federal Reserve, arguably the most powerful institution in the United States. Trump accused Chairwoman Janet Yellen of “doing political things” in keeping interest rates low. While some pundits shrugged off the attack as hot air, Trump’s feelings toward the Fed can no longer be ignored now that he’s President.

That wasn’t the first time Trump took a swing at the Fed. At a rally in September the GOP leader criticized Yellen of doing President Barack Obama’s dirty work.

“She should be ashamed of herself,” he said. “What they are doing is, I believe, it’s a false market.”

Trump’s attacks on the campaign trail was only the start of his cold war with Janet Yellen. Trump may be the most powerful person in the United States, but there are some strings he simply cannot pull. The independent Federal Reserve is one of them.

Yellen made that abundantly clear following the central bank’s decision to raise interest rates in December. In a news conference, Yellen subtly picked apart Trump piece by piece, beginning with the subject of Fed independence.

“I’m a strong believer in the independence of the Fed,” Yellen said following the central bank’s decision to raise interest rates for only the second time in a decade. “We have been given the independence by Congress to make decisions about monetary policy in pursuit of our dual mandate objectives of maximum employment and [minimum] inflation. That’s what I intend to stay focused on, that’s what the committee is focused on.”

She also responded to Trump’s threat that he would force her out of office before her tenure, “I guess the way I think about it is that I was confirmed by the Senate to a four-year term. The term of the Fed chair was not meant to coincide with the president … I do intend to serve out my four-year term.”