WASHINGTON — Under fire for its role in providing fetal tissue for research, Planned Parenthood asked the government’s top health scientists Wednesday to convene a panel of independent experts to study the issues surrounding the little-known branch of medicine.

Planned Parenthood’s request to the National Institutes of Health came as Senate Republicans pressed their fight to bar the organization from receiving federal aid. But likely opposition from at least one Republican senator highlighted the long odds the effort will face in a Senate showdown vote expected early next week.

Sen. Susan Collins, R-Maine, a long-time moderate, said she would oppose the proposal because it would immediately end the federal aid, “and I don’t know how all the patients of Planned Parenthood could be absorbed” by other health care providers. She said such action should await an investigation into Planned Parenthood’s practices.

Planned Parenthood, which gets more than $500 million of its $1.3 billion annual budget from federal and state programs, has been under fire since the release of videos showing some of its officials discussing how they obtain organs from aborted fetuses for research. The videos were furtively recorded by an anti-abortion group.

“The inflammatory and misleading videos have pushed this issue into the national spotlight,” said the letter from Planned Parenthood President Cecile Richards. “And a thoughtful, careful review by leading medical and ethical experts could do a lot to help the public and policymakers think through this issue and reach informed conclusions.”

Anti-abortion groups and many Republican lawmakers have said it is morally wrong for the group to obtain fetal organs for research, and some have accused it of selling the tissue for profit, which would be illegal.

Planned Parenthood says it abides by a law that allows providers to be reimbursed for costs of the procedures, only does it in fewer than five states, and gives the tissue to researchers only with the mother’s consent.

Scientists have conducted fetal tissue research since the 1930s, and use the organs – from miscarriages and abortions – to seek treatment and cures for Alzheimer’s, Down syndrome and other debilitating diseases. The National Institutes of Health spent $76 million on human fetal tissue research last year.

In the late 1980s, a National Institutes of Health panel studying fetal tissue transplants for studying Parkinson’s diseases said the research was acceptable, saying such experiments were promising and did not influence women’s decisions to have abortions.

Amanda Fine, a spokeswoman for the National Institutes of Health, had no immediate comment on Planned Parenthood’s request.

The Senate GOP bill would end federal support for Planned Parenthood and instead distribute the money to community health centers, state and local agencies and other providers who do not perform abortions.

Sen. Joni Ernst, R-Iowa, the bill’s sponsor, said Planned Parenthood’s role in obtaining fetal organs “is morally reprehensible and wrong.” A co-sponsor, Sen. James Lankford, R-Okla., said the videos show what happens when mothers agree to provide fetal tissue for research and “the child that was taken out of them was dismembered and sold for parts.”

Democrats stepped up their defense of Planned Parenthood, focusing on the health-care services the beleaguered group provides for millions of women.

White House spokesman Eric Schultz said the administration strongly disagrees with the bill, adding, “We support them getting the types of services and health care they need.”

No. 2 Senate GOP leader John Cornyn of Texas said health centers in his state “can absorb the legitimate health care needs of women for purposes that we all agree on, primary health care.”

Republicans have 54 Senate seats but will need 60 votes to keep the bill alive. Cornyn said if the bill failed next week, Republicans would likely raise the issue on future legislation.

The dispute spilled over onto the Democratic presidential campaign trail as well.

Front-runner Hillary Rodham Clinton told the New Hampshire Union Leader on Tuesday that the videos were “disturbing,” but also said Planned Parenthood “for more than a century has done a lot of really good work for women.” She said questions “about the whole process” should be addressed broadly, not aimed solely at Planned Parenthood.

Another Democratic presidential hopeful, Sen. Bernie Sanders of Vermont, said attacks on Planned Parenthood were “part of a long-term smear campaign by people who want to deny women in this country the right to control their own bodies.”