Should the government pay for medical research that uses tissue from aborted fetuses? This debate, ever smoldering, has erupted again, pitting anti-abortion forces in the Trump administration against scientists who say the tissue is essential for studies that benefit millions of patients.

In a letter last week that read like a shot across the bow, the National Institutes of Health warned the University California, San Francisco, that its $2 million contract for research involving the tissue, previously renewed for a year at a time, would be extended for only 90 days and might then be canceled.

University scientists had been using fetal tissue to create so-called humanized mice, which can then be used to test drugs and vaccines. The university has played a key role in testing antiviral drugs to treat H.I.V. infection. And researchers say that the mice, which essentially have a complete human immune system, are indispensable.

Scientists who do this work sometimes receive threats, so U.C.S.F. has asked that its researchers not be named for security reasons. The events there were first reported by the Washington Post.