President Trump continued his assault on the chair of the Federal Reserve on Friday, saying he would like Jerome H. Powell to resign. The president, who has berated Mr. Powell for weeks, called the chair a “stubborn mule” who has “Trump derangement syndrome” for his refusal to immediately lower borrowing costs.

“I’d love for him to resign if he wanted to,” the president told reporters in the Oval Office.

Such attacks have become a mainstay in Mr. Trump’s second term in the White House. The president, who elevated Mr. Powell to Fed chair during his first term, has spent the past few weeks castigating him for not moving quickly enough to cut interest rates.

Mr. Trump has long been a fan of low interest rates, which make it cheaper for businesses and consumers to borrow and in turn fuel growth. He cajoled Mr. Powell during his first term when he thought the Fed chair was taking too long to lower rates.

But Mr. Trump’s interest in lower borrowing costs has taken on more significance this time around. He is pushing Republican lawmakers to approve an expensive tax-cut package that would require the United States to sell large sums of debt to finance it.

That goal has become harder — and more expensive — given that interest rates remain elevated in a range of 4.25 to 4.5 percent. Mr. Trump has argued that the government would save “billions” if the Fed lowered interest rates, calling for as much as an immediate 2.5-percentage-point decline.

“We have a guy that’s just a stubborn mule and a stupid person that is making a big mistake,” the president said on Friday. The country is paying more to service its debts “because we have a guy who’s suffering from Trump derangement syndrome, if you want to know the truth,” he added. “He’s not good for our country.”