Mehmet Oz, left, and Herschel Walker are both running for US Senate.
CNN  — 

President Joe Biden has formally asked two Trump-appointed members of the President’s Council on Sports, Fitness & Nutrition to resign from their posts or be terminated.

Herschel Walker and Mehmet Oz, who are now Republican Senate candidates in Georgia and Pennsylvania, respectively, were appointed in 2018 by President Donald Trump to serve on the council. Trump reappointed them to two-year terms in December 2020, shortly before he left office.

It’s against the Biden administration’s policy for federal candidates to serve on presidential boards, according to a White House official. The official said letters to Oz and Walker were sent Wednesday requesting their resignations by 6 p.m.

Walker, the current front-runner for the GOP nomination to take on Democratic Sen. Raphael Warnock in Georgia, said Thursday that he would not be resigning.

“President Biden is so scared about us beating Raphael Warnock that he has asked me to resign from my unpaid position on the President’s Council on Sports, Fitness, and Nutrition,” he said on Twitter. “I’m not a quitter so you are going to have to fire me.”

Oz, who’s seeking the GOP nod in a contentious primary in Pennsylvania, had taken a similar stand the previous day.

“President Trump appointed me to two terms on the President’s Council on Sports, Fitness & Nutrition. The White House just emailed me demanding my resignation by the end of the day,” he said in a statement Wednesday. “Clearly, Joe Biden can’t be around anyone who doesn’t completely fall in line with his fear-mongering authoritarian one-size-fits-all COVID handling. I am proud of my service and will not resign.”

Both Republican candidates face May primaries.

Earlier Wednesday, the White House announced the appointment of chef and humanitarian José Andrés and WNBA star Elena Delle Donne as co-chairs of the fitness council.

Last fall, the Biden White House asked a number of Trump appointees, including former White House press secretary Sean Spicer, senior counselor Kellyanne Conway and national security adviser H.R. McMaster, to resign from their positions on military service academy advisory boards.

CORRECTION: An earlier version of this story mischaracterized how the Hatch Act applies to members of the President’s Council on Sports, Fitness & Nutrition. Oz and Walker were in violation of a Biden administration policy, not the Hatch Act. The story has also been updated with additional reaction.

CNN’s Alex Rogers contributed to this report.