A majority of the Senate, including 14 Republicans, have sent a letter to President Bush asking him to loosen the restrictions on human embryonic stem cell research that he imposed nearly three years ago.

The letter, dated Friday, echoes a similar plea signed last month by 206 members of the House. It may have special resonance during this week of remembrances for Ronald Reagan. Nancy Reagan has been increasingly outspoken in her support for stem cell research, which some experts believe could speed the development of treatments for many ailments, including Alzheimer's disease, the brain-wasting syndrome that took Reagan's life on Saturday.

The field is controversial because obtaining the cells requires the destruction of five-day-old human embryos.

"This issue is especially poignant given President Reagan's passing," said Sen. Dianne Feinstein (D-Calif.), one of five Senate organizers behind the letter. "Embryonic stem cell research might hold the key to a cure for Alzheimer's and other terrible diseases."

Human embryonic stem cells are able to grow into virtually all kinds of tissues and show a promising capacity to help regenerate failing organs. Bush, expressing concern about the ethics of embryo destruction, announced on Aug. 9, 2001, that federal funds could be used to study only those human embryonic stem cell colonies that were established by that date. That has precluded federally funded studies on more recently derived colonies that many scientists agree are more medically promising.

"We would very much like to work with you to modify the current embryonic stem cell policy so that it provides this area of research the greatest opportunity to lead to the treatments and cures for which we all are hoping," the two-page Senate letter concludes. It is signed by 58 senators, including Feinstein and her four fellow organizers: Tom Harkin (D-Iowa), Orrin G. Hatch (R-Utah), Edward M. Kennedy (D-Mass.) and Arlen Specter (R-Pa.).

The letter does not recommend specific policy changes, but it notes that an estimated 400,000 frozen embryos are being stored at U.S. fertility clinics, most of them fated to be discarded. Advocates would like to create a mechanism by which couples could donate unused embryos to federally supported researchers.

The letter notes that the number of available lines under the Bush policy has proved to be far smaller than anticipated. Fewer than 20 such lines are available today, compared with the nearly 80 the government said it had identified early on.

Moreover, the Senate letter says, all those lines have been mixed with mouse cells, compromising their therapeutic potential.

The letter also warns that the United States may be falling behind other countries that are aggressively pursuing stem cell cures, including Britain, Singapore, South Korea and Australia.

The White House has consistently rebuffed suggestions that its policy is inadequate, saying the private sector is welcome to pursue the field -- a position that spokesman Trent Duffy reiterated yesterday. The administration has emphasized the medical potential of adult stem cells, which can be extracted without destroying embryos but which some scientists believe to be less useful medically.

Among the notable signatures on the Senate letter are those of abortion opponents Lamar Alexander (R-Tenn.), Ben Nighthorse Campbell (R-Colo.), Thad Cochran (R-Miss.) and Trent Lott (R-Miss.). Also signing was Sen. Mary Landrieu (D-La.), who had worked closely with Sen. Sam Brownback (R-Kan.) to oppose embryo research but has distanced herself from the Brownback camp since winning a close reelection late in 2002.