The letter was posted on Friday, a day before Ronald Reagan died from the disease, and made public yesterday. Signed by 58 of the 100 senators, it told Mr Bush his policy provided "difficult challenges" to those seeking a cure for Alzheimer's.
"This issue is especially poignant given President Reagan's passing," Senator Dianne Feinstein said. "Embryonic stem cell research might hold the key to a cure for Alzheimer's and other terrible diseases."
Mr Bush has placed restrictions on public money being used to support embryonic stem cell research and opposes using stem cells from most embryos for religious reasons.
A White House spokesman, Ken Lisaius, said Mr Bush stood by his stem cell policy.
"The president remains committed to exploring the promise of stem cell research but at the same time continues to believe strongly that we should not cross a fundamental moral line by funding or encouraging the destruction of human embryos," Mr Lisaius said.
"The president does not believe that life should be created for the sole purpose of destroying it. He does believe we can explore the promise and potential of stem cell research using the existing lines of stem cells."
The senators' letter made no mention of the former Republican president who had a decade-long battle with Alzheimer's, which affects the brain, causing memory loss, confusion, mood changes, hallucinations, speech problems and incontinence.
But the battle to shift the president's stance on the issue is an longstanding one, supported by former presidents Gerald Ford, Jimmy Carter and Bill Clinton.