President George W. Bush planned to cast the first veto of his presidency, yesterday, to stop legislation that would expand federally funded embryonic stem cell research.

The Senate on Tuesday approved the legislation, which has also been passed by the House of Representatives. But neither chamber expects to have the two-thirds majority needed to override Bush’s veto. The stem-cell debate has emerged as an issue in several Senate races in the run-up to congressional elections this November and may be a factor in the 2008 presidential contest.

In 2001 Bush allowed scientists to use 78 stem-cell lines then in existence, most of which proved unsuitable for research. But Bush opposes expanded research because it entails destruction of human embryos.

The new bill would allow federally funded research using excess embryos from fertility clinics, which would otherwise be thrown out. Supporters say they should be used to search for cures for illnesses ranging from diabetes to paralysis.