BBC News Online looks at the state of the stem cell debate in the US, and why it is making an appearance in this final stretch of the race for the White House.

What are stem cells?

Stem cells are master cells that have the ability to develop into any of the body's tissue types.

The late Christopher Reeve was a vocal advocate of stem cell research

Scientists hope that by growing such cells in the laboratory they can programme them to form specific tissue such as kidney, heart or even brain tissue.

This tissue could then be used to treat diseases such as Parkinson's, diabetes and Alzheimer's, or perhaps as an alternative to organ transplants.

Where do stem cells come from?

They can be extracted from human embryos obtained from fertility treatments or abortions. Once isolated, the cells can be grown in the laboratory and stored for future use. Each reservoir of cells, derived from a single embryo, is known as a cell line.

A more reliable supply, however, would be obtained by cloning embryos specifically for their stem cells. One embryo could, therefore, be a source for many thousands of cells.

Where do the candidates stand on this?

On 9 August, 2001, President George Bush declared that federal funding would only be available for research using existing stem cells - meaning that no new lines could be created, whether from existing embryos or cloned embryos.

He maintains this approach, noting that he is the first president to have provided federal funding for this kind of research (to the tune of $216m in 2003) while simultaneously questioning some of the expectations that the research may lead to cures.

"Stem-cell research is in a very early state and while it may hold great promise we should not overstate the state of science, or politicise these issues," he wrote in a recent edition of Nature magazine.

There is no law against private stem cell research in the US, but Mr Bush has called for an international United Nations ban on all human cloning, prompting ire from a number of countries which allow the process under regulated conditions for research purposes.

Democratic challenger Mr Kerry wants the US to join this list of countries. He has said he would "lift the ideologically driven restrictions on stem-cell research by overturning the ban on federal funding on new stem-cell lines - all while ensuring rigorous ethical oversight."

While Mr Bush argues that creating new stem cell stocks would be unethical because it involves the destruction of embryos, Mr Kerry has declared that one would be "respecting life" by using the technology to try to find cures for diseases.

What's the political mileage in it?

There are Catholics and social conservatives who oppose abortion and consequently the destruction of embryos required for stem cell research. Many of these are also supporters of Mr Bush.

Bush has challenged some of the hopes for stem cells

When he announced federal funding with conditions attached in 2001, he was embarking on a delicate balancing act, seeking to placate his anti-abortion supporters without appearing to stand in the way of research. Much of Bush's social policy has been a similar straddle.

The Kerry campaign, however, believes its stance on the issue is one that could move voters - including secular Republicans - in this final stage of the election race, particularly given the sentiment sparked by the recent death of former Superman actor Christopher Reeve.

Reeve was a prominent advocate of stem cell research, and along with other high-profile advocates such as actor Michael J Fox and the Reagans, is seen as having brought the debate on the issue truly into the public domain.