WASHINGTON — In a contentious confirmation hearing to become the nation’s top health official, Robert F. Kennedy Jr. struggled to answer questions about Medicare and Medicaid, programs that affect tens of millions of Americans, or to provide details about how he would work to drive down health care costs.

Kennedy had faced a well-funded opposition campaign from Republicans and Democrats who highlighted his anti-vaccine rhetoric and support of abortion access. Democrats questioned his past remarks, while Republicans praised him for his ideas to reduce food additives and hopes to research a rise in diseases.

As President Donald Trump’s choice to be Secretary of Health and Human Services, he ended Wednesday’s three-hour hearing before the Senate Finance Committee with strong endorsements from Republicans for him to lead the department that accounts for $1.7 trillion in federal spending.

But Kennedy needs backing from nearly all Republicans to land the job. And one key vote from Republican Sen. Bill Cassidy of Louisiana, a physician who heads up the Health, Education, Labor and Pension committee that Kennedy will sit before Thursday, appeared still uncertain.

Kennedy had no answers for Cassidy, who asked the nominee to share plans for Medicaid reform, a multibillion-dollar taxpayer-funded program that covers health care for about 80 million people, including children. Republicans have said they might need to make deep cuts to Medicaid to fund Trump’s proposals.

After Cassidy pressed him several times, Kennedy finally replied, “I don’t have a broad proposal for dismantling the program.”

He went on to misstate how the biggest programs at the agency work, eliciting corrections from senators throughout the hearing. Medicaid, he wrongly said, is fully paid for by the federal government — it’s not; states and federal taxpayers fund it. Later, he talked about Medicaid’s “high premiums and high deductibles,” although the virtually-free program has neither. He appeared confused about questions on the hundreds of community health clinics the agency funds in cities across the country or the role it plays in a federal law that mandates emergency rooms to stabilize anyone who presents at their facility.

And on some of the most controversial questions — like his plans for abortion — he deferred to Trump. Kennedy, a longtime Democrat, had previously said during his failed presidential bid that he supported access to abortion but on Wednesday he said that every abortion is a “tragedy.”

Views on vaccines still the biggest issue for Democrats

Kennedy tried to assure senators that he supports childhood vaccines, pointing out that his children are vaccinated. “I believe that vaccines play a critical role in health care,” Kennedy told the committee.

Republicans didn’t ask about Kennedy’s views on vaccines. Democrats, though, repeatedly brought up previous remarks or evidence that Kennedy has discouraged their use. The accusations often led to a terse back-and-forth and outbursts from the audience.

Democrats also pointed to Kennedy’s writings, podcast appearances and other comments to show that Mr. Kennedy has embraced conspiracy theories and unproven alternative remedies, especially when it comes to the safety and efficacy of vaccines. Sen. Michael Bennet of Colorado pressed Kennedy over controversial and false claims he’s made about COVID-19, pesticides and AIDS. Kennedy acknowledged some of the statements and denied others.