This will be the third veto of Bush's presidency. His first occurred last year when he rejected legislation to allow funding of additional lines of embryonic stem cells - a measure that passed over the objections of Republicans then in control. Earlier this year, he vetoed legislation that would have set timetables for US troop withdrawals from Iraq.

Opponents of the latest stem cell measure insisted that the use of embryonic stem cells was the wrong approach on moral grounds - and possibly not even the most promising one scientifically. They cite breakthroughs involving medical research conducted with adult stem cells, umbilical cord blood and amniotic fluid, none of which involve the destruction of a human embryo.

The science aside, the issue has weighty political implications.

Public opinion polls show strong support for the research, and it could return as an issue in the 2008 elections.

Democratic presidential candidate Hillary Rodham Clinton appeared in Hanover, N.H., this week with a child who has diabetes and a paralyzed 23-year-old to urge Bush not to veto the bill. Last month, the issue was a topic at a debate with Republican presidential hopefuls in California.

The bill Bush is vetoing passed Congress on June 7, drawing the support of 210 House Democrats and 37 Republicans. That was 35 votes fewer than needed to override a veto. The Senate cleared the bill earlier by a margin that was one vote shy of the two-thirds needed to overcome Bush's objections.

According to the National Institutes of Health Web site, scientists were first able to conduct research with embryonic stem cells in 1998. There were no federal funds for the work until Bush announced on Aug. 9, 2001, that his administration would make the funds available for lines of cells that already were in existence.

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