(WASHINGTON) -- Dr. Heidi Overton is appearing before a Senate committee on Thursday for her confirmation hearing as the next commissioner of the Food and Drug Administration.

Overton was nominated in August by President Donald Trump, who described her in a post on social media as a "ROCKSTAR in my Administration."

She is sitting before the Senate Health, Education, Labor, and Pensions (HELP) committee, where she is likely to face questions about childhood vaccines and the abortion drug mifepristone.

If confirmed, Overton will be the second person to lead the FDA after the previous commissioner, Marty Makary, resigned following public clashes with lawmakers, major pharmaceutical companies and Trump over fruit-flavored vapes.

Kyle Diamantas has served as acting FDA commissioner since May 2026.

Sen. Bill Cassidy, R-La., chair of the HELP committee, began the hearing by asking Overton if the measles, mumps, rubella (MMR) vaccine Is lethal. Overton replied it is not.

When asked if the vaccine is safe and effective, Overton reiterated that the vaccine is safe and effective against measles.

"Are there any vaccines on the U.S. market right now which are unsafe?" Cassidy asked.

"Senator, every vaccine on the U.S. market right now is safe and effective, according to the FDA," Overton said.

Cassidy attempted to press Overton on whether she disagreed with the president on the safety and efficacy of the MMR vaccine, but Overton deflected from answering directly.

Sen. Patty Murray, D-Wa., asked Overton if she believes mifepristone is safe and effective. Kennedy launched an FDA safety review of the drug last year.

"I know that it has met the safety and efficacy standard according to the FDA review," Overton replied.

Murray expressed concern over an article she said Overton published three years ago, which she said described medication abortion as dangerous to women.

She also asked if Overton believed an IUD is an abortifacient, which is a substance used to induce an abortion or terminate a pregnancy. Overton did not answer directly.

"If I'm confirmed as FDA commissioner, I'll be fully briefed on all their prior decisions and make determinations according to the standards of safety and efficacy," Overton said.

She has espoused anti-abortion views and supported reductions to the childhood immunization schedule. At the executive order signing, she touted the use of what she called "gold-standard science" and restoring vaccination authority to parents.

Overton was nominated for the job, in part, because she is a loyalist of the president, two sources familiar with the nomination tell ABC News.

She's been described as a loyal staffer, particularly in the presidents' efforts to overhaul the childhood vaccine schedule, according to a former senior administration official.

"Why would you not want your own loyalist in there?" the source said about Trump's choice, adding "She [Overton] kind of goes along with things." The former official supports Overton but has previously acknowledged that, "She's been in the world of politics for a long time and understands everything's about loyalty."

Overton is a physician with a doctor of medicine degree from the University of New Mexico and also holds a doctorate from Johns Hopkins University.

Prior to her role as deputy assistant to the president for domestic policy, she was vice chair of the America First Policy Institute's (AFPI) Center for a Healthy America, where Overton was also the chief policy officer, a nonprofit that aims to advance policies that it says put the American people first. AFPI was run by Agriculture Secretary Brooke Rollins before Rollins was tapped to join the president's Cabinet.

Several Trump administration officials held positions there before he was reelected, including Education Secretary Linda McMahon.

Prior to the hearing, Cassidy expressed "strong concerns" about Overton's nomination because of her participation in the White House executive order overhauling the childhood vaccine schedule.

"That alone is almost disqualifying," Cassidy wrote in a post on X.

Cassidy, a physician and liver doctor, has long championed the efficacy of vaccines and has grilled Trump's health officials on their willingness to stand up to Health and Human Services Secretary Robert F. Kennedy Jr., including recently installed director of the Centers for Disease Control and Prevention Dr. Erica Schwartz.

In a statement to ABC News, Democratic Women's Caucus Chairwoman Teresa Leger Fernández slammed the nomination.

"I call on my colleagues in the Senate to reject her nomination," Fernández wrote, adding "Overton has publicly denied scientific fact and repeatedly launched conspiratorial attacks against our right to abortion access, the safety of mifepristone, and Planned Parenthood."

Fernández added: "She is not qualified to serve as FDA Commissioner, and her extremist beliefs and loyalty to the Trump administration will put the lives of millions of women at risk."

In a statement, Dr. Amesh Adalja, senior scholar at the Johns Hopkins University Center for Health Security, also criticized her nomination.

"The fact that Dr. Overton played a central role in an executive order that shrinks the childhood vaccine schedule and calls for splitting MMR into shots that don't exist, all without new evidence, calls into question her scientific judgement," Adalja said. "The fact that she has garnered the praise of the nation's chief anti-vaccine propagandist, Secretary Kennedy, is also concerning."

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