WASHINGTON — At least 16 of the 78 human stem cell colonies approved by President Bush for federal research money have died or failed to reproduce in their laboratory dishes — making them useless to scientists — and most of the others are unlikely ever to become available for disease research, according to interviews and a new analysis by the National Institutes of Health.

The unpublished NIH analysis, circulating Tuesday on Capitol Hill, said only about a quarter of the Bush-approved cell colonies are ever likely to be available, far fewer than supporters of the president’s policy had predicted.

Moreover, several of the Bush-approved colonies already available to researchers are beginning to show genetic abnormalities, potentially undermining their medical usefulness, researchers said.

Advocates of stem cell research, who believe it offers the possibility of curing a range of diseases from diabetes to Parkinson’s, said these developments confirmed fears they expressed in 2001, when Bush announced he would allow federal funding only for stem-cell colonies that had already been extracted from human embryos as of Aug. 9 of that year.

A bipartisan group of House members — including some Republicans who until now accepted the Bush policy — are gathering signatures on a letter of their own calling for a policy change.

Sensing that the tide may be shifting in their favor, scientific organizations have stepped up their campaign to ease restrictions on the controversial research, which makes use of embryos slated for destruction by fertility clinics.

"I think the administration has been trying to implement the existing policy in good faith," said Lawrence Soler of the Juvenile Diabetes Research Foundation, which supports expanding federally funded research. "I think it’s just come to a point now of having to face that we’re not as far as we had hoped we’d be — or even, we believe, where the administration had hoped we’d be."

The administration said it was planning no change of policy.

The debate centers on a policy that has been among the most contentious of Bush’s tenure. Democrats have generally been united in supporting broad research on embryonic stem cells, while the Republican majority in Congress has been sharply divided.

Scientists are excited about the cells because, unlike most adult cells, they can morph into nearly any tissue in the human body. Researchers hope to grow large numbers of cells in the laboratory and then coax them into becoming brain cells that might cure Parkinson’s disease, pancreatic cells to cure diabetes, and so on.

But creating a laboratory colony of stem cells requires destroying a 5-day-old human embryo. Social conservatives have opposed the work, saying embryo destruction is tantamount to murder.