President Bush restated his commitment to sharply limit stem-cell research, bucking renewed pressure from Nancy Reagan and others to loosen the restrictions in the aftermath of the death of former President Ronald Reagan. Some believe that research using stem cells could result in treatments for Alzheimer's, the disease that led to Mr. Reagan's death, and Mrs. Reagan has helped lead the fight to ease the restrictions Mr. Bush imposed in August 2001. Fifty-eight senators recently signed a letter asking Mr. Bush to reconsider the rules.