George Bush was poised today to overrule senior Republicans, along with the vast majority of Americans, by using his presidential veto for the first time to defeat a bill on stem cell research. The bill would expand US government funding to include embryos which did not exist prior to August 2001, when Mr Bush announced a moratorium. Scientists working in the field say newer stem cell lines are essential to realise the promise of a cure for Alzheimer's, Parkinson's, diabetes, and some cancers.But the president is using his first veto - having assented to 1,116 other laws passed by Congress during his time in office - in a high-stakes strategy that is likely to appeal to religious conservatives but risks splitting his party in advance of November's midterm elections.