This paper examines the influences that congressional staff people viewed as important in shaping legislators' voting decisions on the human embryonic stem (ES) cell research bill in the 109th Congress, the first legislation vetoed by President George W. Bush. The analysis illuminates factors that impact congressional decision making on a salient issue with a strong moral component. Constituent concerns, ideology, and a desire to make good public policy all centrally affected members' choices; however, moral overtones permeated considerations relevant to the human ES cell research question. In addition, at least three influences that directly reflect or relate to members' moral claims — religious convictions, personal connections to potential beneficiaries of human ES cell research, and moral pressure from outside interests — were important also. The analysis draws on data gathered from interviews with congressional aides.