MADISON, Wis. – Stem cell experts said Monday they hope the next U.S. president will end political curbs on embryonic stem cell research but some worry recent comments by Republican candidate John McCain suggest his past support for such research may be waning.

Both McCain and Democrat Barack Obama have said they favor easing restrictions on spending public money to finance embryonic stem cell research.

“My hopes are that politics get out of it,” University of Wisconsin-Madison stem cell researcher James Thomson told reporters. “It's been a long eight years,” he added, referring to President Bush's staunch opposition to such research.

Stem cells taken from balls of cells that develop days after conception offer promise for regenerative medicine because they give rise to all tissues in the body.

But some oppose their use because they involve destruction of the embryo.

In 2001, Bush issued an executive order allowing only limited federal funding of work involving human embryonic stem cells. Congress has tried several times to loosen restrictions but Bush has vetoed every effort.

Obama supports embryonic stem cell research. McCain's stand is less clear.

“While I support federal funding for embryonic stem cell research, I believe clear lines should be drawn that reflect a refusal to sacrifice moral values and ethical principles for the sake of scientific progress,” McCain said in a statement given Sept. 15 to a website devoted to discussing science and the elections, www.sciencedebate2008.com/.

“Moreover, I believe that recent scientific breakthroughs raise the hope that one day this debate will be rendered academic.”

Thomson said he was concerned about McCain's comments.

He said the worst thing that could happen is that McCain ”gets painted into a corner” and makes a promise to oppose embryonic stem cell research that he would be forced to follow through on if elected president.

“It's too important to make it a Republican or Democratic issue,” Thomson told reporters at the World Stem Cell Summit in Madison.

Both Thomson and Wisconsin Gov. Jim Doyle told reporters they were concerned about efforts by opponents of embryo research to offer stem cell advances as a reason to discontinue funding for embryonic stem cell research.

Researchers, including Thomson, have discovered ways to make powerful stem cells using ordinary skin cells. Called induced pluripotent stem cells or iPS cells, these new cells appear to behave like embryonic stem cells.

Other teams have directly reprogrammed one type of cell into another type, and other types of stem cells have also shown promise in experiments.

Thomson said that while promising, it is not clear if these cells are useful as human embryonic stem cells.

“Although I personally believe the future is in iPS cells, I could be wrong. There could be evidence to show these cells are fatally flawed. We have to keep an open mind,” Thomson said.

(Editing by Maggie Fox and Philip Barbara)