Dr. Mehmet Oz, who is President Donald Trump’s nominee to lead the Centers for Medicare and Medicaid Services, may have “significantly underpaid his Social Security and Medicare taxes” in recent years, according to a memo prepared by Democratic staff on the Senate Finance Committee and obtained by CNN.

Oz, who is appearing before the committee Friday morning for his confirmation hearing, paid “negligible” Social Security or Medicare taxes in 2022 and no such taxes in 2023 – avoiding $440,000 in levies, according to the memo and Sen. Ron Wyden, the committee’s ranking member.

The celebrity doctor would be responsible for overseeing Medicare if he is confirmed. CMS provides health care coverage to more than 160 million Americans through Medicare, Medicaid and the Affordable Care Act health insurance.

Oz took the position that he was not liable for certain taxes – including a self-employment tax, known as SECA – on income from his media entity Oz Property Holdings, the memo said.

Oz’s accountant told the committee’s Democratic staff that the doctor was relying on the limited partner exception, which allows certain income to be excluded from self-employment earnings. But Democratic staffers contend that he was actively involved in the company that bears his name, noting that “Dr. Oz’s position is counter to the position of the Department of Treasury.”

A spokesperson for Oz pushed back on the staff’s charge, saying the “Office of Government Ethics has conducted an extensive review of Dr. Oz’s finances as part of the regular vetting process” and “OGE has transmitted to the Senate a letter indicating that any potential conflicts have been resolved.”

The White House did not immediately return a request for comment.

CNN’s Veronica Stracqualursi contributed to this report.