ASHINGTON, Feb. 22 — With the resignation of the head of the Centers for Disease Control and Prevention on Thursday, four of the most important federal health jobs are now vacant. Experts say there is a vacuum in health leadership as the Bush administration is trying to improve the nation's defenses against bioterrorism.

But because the jobs require such a high level of expertise, filling them is immensely complicated. The politics of the nominations are sensitive for a president who has taken strong stands against abortion and for limits on embryo research and whose conservative base expects his appointments to reflect those views.

Already, the abortion issue may have clouded the prospects of one candidate, Dr. Anthony S. Fauci, to become head of the National Institutes of Health. Dr. Fauci, the first choice of Tommy G. Thompson, the health and human services secretary, was recently told that he was highly unlikely to be nominated. Officials said the White House did not want to grant Dr. Fauci's request to keep his current job, as chief of a branch of the institutes, while serving as director.

Conservatives say there may be another reason. In 1988, Dr. Fauci made remarks supporting research that uses tissue from aborted fetuses. Unlike the president, Dr. Fauci has painstakingly refused to state his position on abortion.

"Social conservatives have objected to him," said Deal Hudson, editor of Crisis magazine, a Roman Catholic monthly.

Mr. Hudson, who has advised the administration on courting the Roman Catholic vote, said White House officials had assured him that "they are looking for somebody with the research credentials and the ethical outlook that the president has espoused very clearly."

With Dr. Fauci apparently out of the running, one administration official said, the White House is now considering Dr. Elias Zerhouni, executive vice dean of the Johns Hopkins University School of Medicine. A potential complication, though, is that Hopkins has been a pioneer in embryonic stem cell research, and Dr. Zerhouni was instrumental in creating a new institute at the university to help advance that work and has promised to "intensely pursue this research because of its huge potential impact on human health."

Embryo research has been a delicate issue for Mr. Bush. In August, he approved federal financing for certain studies of stem cells derived from human embryos, but imposed strict limits. The president also opposes so-called therapeutic cloning, which involves creating embryos to obtain stem cells that might be used to treat disease.

Dr. Zerhouni has not been in the public spotlight in these debates, and his opinions are not widely known. But conservatives have made clear that they expect any candidate to share their views. Any candidate will inevitably be asked about them in the Senate confirmation hearings, or perhaps even before.

That was the case with Dr. Fauci. Late last year, Senator Sam Brownback, the Kansas Republican who is an ardent opponent of stem cell research, summoned Dr. Fauci to his office and asked his opinion on issues like therapeutic cloning. A person familiar with the meeting said Mr. Brownback made it clear he was not satisfied with Dr. Fauci's answers.

In addition to the N.I.H. opening, the job at the disease control centers will become open at the end of next month, when Dr. Jeffrey P. Koplan, the director, steps down. The administration also lacks a food and drug commissioner, and Dr. David Satcher, the most recent surgeon general, left the administration this month when his term expired.

Each of the positions carries political overtones, and various presidents have had the experience of regretting the people they picked. Dr. C. Everett Koop, the former surgeon general who was chosen for his anti-abortion views, irked President Ronald Reagan by speaking bluntly about AIDS. Dr. David A. Kessler, who ran the Food and Drug Administration under President George Bush, angered conservatives when he took on the tobacco industry.

"It's like at the Supreme Court," said Grover Norquist, president of Americans for Tax Reform and a conservative adviser to the White House. "Everyone wants to act like it's nonpoliticized. But each of these jobs is so politicized that the Democrats in the Senate and the administration are at loggerheads."

That has clearly been the case with the search for an F.D.A. commissioner. When the administration put forth the name of a biotechnology lawyer, Michael J. Astrue,

Democrats objected, saying they they would not confirm someone from industry. So the administration turned to a drug safety expert, Dr. Alastair J. J. Wood of Vanderbilt University.

But Dr. Wood drew complaints from pharmaceutical executives, who contributed heavily to Mr. Bush's campaign. A spokesman for Vanderbilt said this week that Dr. Wood had been told by the White House that he was no longer being considered for the position.

Health experts, meanwhile, say it is becoming increasingly important for the administration to fill the positions. Mr. Bush has proposed a vast increase in spending to prepare for bioterrorism, and the nation's health agencies are taking on an important homeland security role.

"I think the White House has got to focus whatever attention that is needed to make this happen," said Gail R. Wilensky, who ran the Health Care Financing Administration under President George Bush. "It appears that too much of the subcabinet leadership in health is missing."

The vacancies have also become a source of personal frustration to Mr. Thompson, who was said to be particularly upset that the White House had rejected Dr. Fauci's candidacy, according to officials at the health agency.

"Tommy has gone to bat for him," an aide to Mr. Thompson said. "The secretary wants him really badly."

In a brief interview this week, Mr. Thompson described himself as a huge fan of Dr. Fauci and insisted that Dr. Fauci's views on fetal tissue research or abortion were "not part of any consideration."

A spokesman for the secretary, Kevin Keane, said he was confident that the White House would nominate some candidates soon. "Ultimately," Mr. Keane said, "when it is said and done, there are going to be very strong leaders in those positions. We have confidence in that."