WASHINGTON  Despite Sen. John McCain's recent votes in favor of embryonic stem cell research, some advocates are worried he wouldn't be as supportive in the Oval Office as he was in the Senate chamber.

The Republican presidential nominee has twice supported legislation vetoed by President Bush that would have lifted limits on federal grants for such research.

But advocates fear the 2008 Republican Party platform's outright ban on the research, his running mate Sarah Palin's opposition and recent statements from McCain signal his support could waver.

This week, campaign spokesman Ben Porritt cited McCain's Senate voting record but would not specify whether a McCain-Palin administration would support federal funding for embryonic stem cell research. He said only that they would "support stem cell research," which could refer to amniotic or reprogrammed adult stem cells.

A radio advertisement his campaign released Friday called "Stem Cell" says McCain would lead his congressional allies to improve America's health with medical breakthroughs but doesn't mention the word "embryonic."

"We've always been a little concerned that Senator McCain may have voted for this, but would he be there in the long term when he is the president of the United States versus just a senator?" said Rep. Mike Castle, R-Del., who supports McCain but advocates the research.

"It's sort of been a nagging concern all along and it's just been heightened a little bit by both the platform and Governor Palin's involvement," Castle said.

Those who oppose the research are also uncertain.

"He seems to be moving away from that position, but we currently have not heard he's changed that position," said Colleen Parro, executive director of the Republican National Coalition for Life.

The issue could be tricky for McCain as he tries to appeal to conservatives while reaching out to independents. Embryonic stem cell research played an important role in the 2006 elections, with 58% of the races between an opponent and supporter of stem cell research going to the supporter, according to the Center for American Progress.

Polls show most Americans support research using embryonic stem cells, which can grow into any cell in the body and could potentially be used to help treat people with cancer, Parkinson's and other diseases. Scientists wish to explore different forms of stem cells to see what works best, but some religious groups oppose embryonic research because it destroys the days-old human embryo.

McCain's presidential campaign website states that he would "strongly support" funding for amniotic fluid and adult stem cell research and other scientific studies "that do not involve the use of human embryos."

Porritt did not respond when asked Palin's view, but a Nov. 5, 2006, story in the Anchorage Daily News said she was opposed to embryonic stem cell research.

The issue surfaced on the campaign trail Tuesday when Democratic vice presidential candidate Sen. Joe Biden questioned why Republicans who say they care about children with disabilities don't support stem cell research. Democratic nominee Sen. Barack Obama supports embryonic stem cell research.

Porritt called the statement "a new low" and accused Biden of "launching an offensive debate over who cares more about special needs children." Some McCain supporters called Biden's statement an unfair attack on Palin, the mother of a baby with Down syndrome who pledged to be an advocate for parents of children with special needs.

But an Obama spokesman called it a "substantive difference" and pointed to the Republican Party platform taking "an even more extreme position" on stem cell research than Bush. His 2001 executive order limited federal funding for research on human embryos to a few dozen lines of cells already being studied. The Republican platform bans "the creation of or experimentation on human embryos for research purposes."