The ban, known as the Dickey-Wicker amendment, first became law in
1996, and has been renewed by Congress every year since. It
specifically bans the use of tax dollars to create human embryos — a
practice that is routine in private fertility clinics — or for
research in which embryos are destroyed, discarded or knowingly
subjected to risk of injury.

At first, the ban stood in the way of taxpayer-financed embryonic stem
cell research, because embryos are destroyed when stem cells are
extracted from them. But in August 2001, in a careful compromise,
President Bush opened the door a tiny crack, by ordering that tax
dollars could be used for studies on a small number of lines, or
colonies, of stem cells already extracted from embryos — so long as
federal researchers did not do the extraction themselves.

On Monday, Mr. Obama will throw open the door much farther with an
executive order that will “make clear that the government intends to
support” human embryonic stem cell research, said Harold Varmus, the
president of Memorial Sloan-Kettering Cancer Center, who advises Mr.
Obama on science matters.

To the delight of patients’ groups and scientists, the order will
allow research on hundreds of stem cell lines already in existence, as
well as ones yet to be created, typically from embryos left over from
fertility treatments that would otherwise be discarded.

The order comes just in time for researchers to take advantage of
money in Mr. Obama’s economic recovery package and use it for stem
cell studies. But because of the Dickey-Wicker amendment, federal
researchers would still be unable to create their own stem cell
lines.

Mr. Obama has no power to overturn the Dickey-Wicker ban. Only
Congress, which attaches the ban to appropriations bills, can overturn
it. Mr. Obama has not taken a position on the ban and does not intend
to, Melody C. Barnes, his chief domestic policy adviser, said Sunday.
The president believes stem cell research “should be done in
compliance with federal law,” she said, adding that Mr. Obama
recognizes the divisiveness of the issue. “We are committed to
pursuing stem cell research quite responsibly but we recognize there
are a range of beliefs on this,” she said.

Because embryonic stem cells are capable of developing into any type
of cell or tissue in the body, many scientists and advocates for
patients believe they hold the possibility for treatments and cures
for ailments as varied as diabetes and heart disease. Some researchers
say stem cells may someday be used to treat catastrophic injuries,
like damage to the spinal cord.

Mr. Bush twice vetoed legislation that would have expanded his 2001
policy. Although Mr. Obama’s action on Monday has broad bipartisan
support, it could still be overturned by a successor so House
Democrats are expected to draft legislation that would codify the
president’s executive order.

But with Mr. Obama revisiting the Bush policy, Representative Diana
DeGette, Democrat of Colorado and a leading advocate for embryonic
stem cell research, said Sunday in an interview that overturning the
embryo experiment ban might not be as far-fetched as some critics
imagine.

Ms. DeGette said the first move for lawmakers would be to turn the
steps Mr. Obama takes by executive order on Monday into law. But she
said she was also talking to her colleagues about overturning the
broader Dickey-Wicker restriction.

“Dickey-Wicker is 13 years old now, and I think we need to review
these policies,” Ms. DeGette said. “I’ve already talked to several pro-
life Democrats about Dickey-Wicker, and they seemed open to the
concept of reversing the policy if we could show that it was necessary
to foster this research.”

A senior House Democratic leadership aide, who was not authorized to
speak publicly about the issue, said overturning the ban “would be
difficult, but not impossible,” adding, “It’s not something that we
would do right away, but it’s something that we would look at.”

Fertility researchers also believe the climate is ripe to allow
federal money for their work, especially in light of the recent
controversy over the birth of octuplets in California, said Sean
Tipton, a spokesman for the American Society for Reproductive
Medicine.

“I think we’re thrilled that the president is going to lift the
restrictions on embryonic stem cell research,” Mr. Tipton said Sunday.
“It is clear, though, that Congress needs to remove the restrictions
it puts on other forms of embryo research.”

Already abortion opponents are bracing for a battle. “The
administration now steps onto a very steep, very slippery slope,” said
Douglas Johnson, legislative director for the National Right to Life
Committee. “Many researchers will never be satisfied only with the so-
called leftover embryos.”

One Republican lawmaker, Representative Christopher H. Smith of New
Jersey, is calling Mr. Obama “the abortion president,” and is planning
his own event on Monday to protest Mr. Obama’s new stem cell policy.

Mr. Smith said in an interview Sunday that he did not think lawmakers
would go along with overturning the embryo experiment ban.

“I don’t think it will fly because the movement in the country is in
favor of life,” he said. “For Congress to say that the new guinea pig
will be human embryos, most Americans will find that highly
offensive.”

Mr. Obama’s announcement on Monday will be part of a broader
initiative to make good on his pledge to separate science and
politics. Dr. Varmus, a former director of the National Institutes of
Health who is a co-chairman of a panel that advises Mr. Obama on
science issues, said the president would issue a memorandum to
“restore public confidence in the process by which scientific policy
is used to guide government action,” by directing his administration
to draft guidelines for the use of scientific information and the
appointment of outside science advisers.

In reversing the stem cell policy Mr. Bush put in place in August
2001, Mr. Obama will direct the National Institutes of Health to come
up with new stem cell research guidelines within 120 days.

Ms. DeGette said she is already talking to the White House about what
the bill might say.

“It’s a wonderful development tomorrow,” she said, “but it’s really
the first step in opening up ethical cell-based research.”