Saul was fired after refusing a resignation request, White House officials said. His deputy, David Black, who was also appointed by former President Donald Trump, resigned on Friday on request.

Biden has appointed Kilolo Kijakazi, the current deputy commissioner for retirement and disability policy, to serve as interim commissioner until a permanent candidate is selected.

But Saul said in an interview on Friday afternoon that he would not quit his post, challenging the legality of the White House’s decision to oust him. As the head of an independent agency whose leadership does not normally change with a new administration, Saul’s six-year term was expected to last until January 2025. The White House said a recent Supreme Court ruling gave the president the power to replace him.

Saul challenged it. “I consider myself to be the term social security commissioner,” he said, adding that he planned to return to work Monday morning, logging in remotely from his New York home. He called his ouster a “Friday night massacre.”

“It was the first time that I or my deputy knew this was going to happen,” Saul said of the email he received from the White House personnel office on Friday morning. “It was a lightning bolt that nobody expected. And right now, that leaves the agency in complete turmoil.

Saul’s sacking came after a tumultuous six-month tenure in the Biden administration in which advocates for the elderly and disabled and Democrats on Capitol Hill lobbied the White House to fire him. He had clashed with unions representing his 60,000 employees, who said they had used anti-union tactics. Angry advocates say he hung around as millions of Americans with disabilities waited for him to turn the cases over to the Internal Revenue Service to release their stimulus checks – and accused him of a campaign too zealous to require people with disabilities to restore their eligibility for benefits.