The proposal, which is yet to be drafted into a bill, would ban scientists from using any tissue that comes from an embryo of a legally determined age. Lawmakers have not yet agreed what that age will be. But Rep. David Weldon, R-Fla., suggested it would likely be set at 14 days after conception.

The bill would join a host of other research ethics bills awaiting Senate debate. Last May, the House passed a bill lifting a White House ban on federal funding of research using human embryonic stem cells.

That bill has broad support in the senate but has yet to receive a vote because senators have been unable to agree on ground rules for debate. Stem cell research supporters want to debate the House bill alone, while opponents have demanded that the senate also take up bills banning stem cell research, cloning, research on human-animal hybrids, and other issues.

Weldon said in an interview that he's making the proposal in an effort to win over senators who don't support banning stem cell research.

"I can't get the votes in the Senate, so I may as well try this," said Weldon, who is a physician. "I think it's a pretty clear issue that we don't want to be experimenting on fetuses."

Researchers have pegged embryonic stem cells as a promising method of tissue engineering that could help cure degenerative illnesses like Parkinson's disease and diabetes. Some religious groups oppose the research because it requires the destruction of a human embryo, which many consider tantamount to abortion.

Several states, including Maryland, California, and Massachusetts, now have laws funding stem cell research. But a federal policy established by President Bush in August 2001 limited government funding to research on 77 stem cell lines that already existed at the time.

Many scientists decried the ruling, arguing that the lines are of poor quality and suffer from contamination.

Sen. Sam Brownback, R-Kan, the Senate's leading opponent of embryonic stem cell research, said that researchers are not yet attempting to create fetuses for the sake of extracting tissue, a prospect he described in an interview as "ghastly".

"I want to stop it before it starts," he said.

It remains unclear whether the Senate will debate stem cell legislation before the mid-term elections in November. Brownback's opponents on the issue dismissed the proposal as an attempt to cloud debate on embryonic stem cell research, which enjoys broad public backing.

In an interview, Sen. Orrin Hatch, R-Utah, a pro-life lawmaker who supports the studies, said his senate bill expanding stem cell research already contains language limiting cell extractions to embryos.

Hatch repeated claims that his bill has the 60 votes needed to stave off delaying tactics from opponents. "They can't win on the stem cell issue, so they want to try to muddy it up and peel people off," he said.

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