A measure to expand federal funding of stem cell research has stalled in the Senate but backers unable to get the anticipated July vote instead vowed on Thursday to force the issue one way or another this year.

Despite a veto threat by President Bush, the embryonic stem cell bill cleared the House in May with a surprisingly broad bipartisan margin. Backers believed they had momentum in the Senate and a vote was tentatively set for this month.

But now bill sponsors say there is only the slimmest of chances that the Senate can take up the bill before breaking for its August recess.

It bogged down in a procedural morass involving a half-dozen other stem cell and cloning bills — some written with the apparent aim of peeling away support from the House-passed legislation.

“I think there has been an effort to obfuscate the House-passed bill with a collection of other bills,” said Sen. Dianne Feinstein, a California Democrat and leading advocate of the research.

But they create ethical controversy because human embryos are destroyed when the cells are extracted.

In August 2001, Bush allowed research on a limited number of existing stem cell lines, most of which proved unsuitable and all of which turned out to be contaminated with mouse cells. But he has opposed further research on ethical grounds.

The House-passed bill would permit federally funded research on cells extracted from frozen embryos left over from in vitro fertilization treatments.

Backers, including some who normally oppose abortion, believe it is ethical to conduct potentially life-saving research on embryos that would otherwise become medical waste. There are currently about 400,000 leftover embryos.

If Frist doesn’t broker an agreement, Pennsylvania Republican Sen. Arlen Specter said that as chairman of a health spending panel, he will bring up stem cells when that must-pass appropriations bill is debated after the summer break.

“I’ll bring it up as the first amendment out of the box,” Specter said. “We’ve waited long enough.”

Eric Ueland, a senior Frist aide, said “The leader hopes to call this up and deal with it in a clean and rational manner and will continue to work to that end.” But he acknowledged that a vote next week is looking much less likely.

“It’s not looking good. Both sides have the other by the jugular vein,” said Oregon Republican Gordon Smith, a strong supporter of the embryonic stem cell bill. Asked if he thought the issue would be back in the fall, he said, “Exactly.”

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