This article by Susan Saulny in the New York Times over the weekend notes the limited visibility of embryonic stem cell research as a campaign issue in the Missouri Senate race, where incumbent Republican Jim Talent clearly opposes it and Democratic challenger Claire McCaskill clearly supports it. Theres a constitutional amendment on the November ballot which would unambiguously legalize such research in Missouri, but Saulny reports that neither candidate is highlighting their position on this issue in stump speeches or paid advertising. The reason, Saulny argues, is that the election is very close and both candidates need votes from outside their traditional party bases to win. 
McCaskill needs votes from rural southwestern Missouri (labeled as Ashcroftland by some observers), where voters are generally opposed to the stem cell amendment, and Talent needs votes from urban areas, which are more supportive. In this setting, neither candidate has any incentive to highlight their differences on this issue for fear of alienating voters whose support they need.
Saulny notes that embryonic stem cell research is a more visible issue in other races. In the Maryland Senate race where there is no incumbent, Democratic Representative Benjamin Cardin is highlighting differences in stem cell research policy with Lt Governor Michael Steele, who is the Republican candidate. Steele created a flap early in the campaign when he likened stem cell research to Nazi science experiments (he apologized profusely immediately afterwards) but is running hard to appeal to black voters, a traditional part of the Democratic base (Steele is black). As weve noted before, Governor Jim Doyle of Wisconsin is also pushing stem cell research as a major economic development effort in his re-election campaign.
Other races where stem cells have been more visible seem to be suburban seats that have traditionally been Republican, but are more moderate socially than the national Republican base. Republicans in these districts are frequently split between pro-business groups and social conservatives, and stem cells are a major dividing line in this split. In this sort of environment, playing up differences over issues like stem cells and even abortion may allow Democrats to appeal to independents and even moderate Republican voters. Two districts where this issue has surfaced recently are the 8th district in Illinois (northern Chicago suburbs) and the 25th in New York (which contains Syracuse and environs). In the Illinois 8th, incumbent Democrat Melissa Bean has started running ads highlighting the opposition of her opponent, Republican David Sweeney, to stem cell research. Bean knocked off a long-serving Republican incumbent to win election two years ago, and is receiving significant financial backing from the US Chamber of Commerce and other business groups. In the New York 25th, a Washington issues group called Majority Action is sponsoring a provocative television ad (which you can see in Art Caplans entry below) attacking Republican incumbent Jack Walshs position on stem cell research. Walsh has objected strenuously to the ad, claiming it misrepresents his position. Walsh, one of the more senior Republicans in the House, has historically been very popular in the districthe didnt even have a Democratic opponent in 2004but the race this time is more competitive.
The Missouri Senate race provides evidence for one traditional argument about the effects of party competition on candidate positionthat it leads candidates in close elections to blur differences because they fear they may lose votes if they distinguish themselves from each other too sharply. The suburban House races provide evidence of the other viewthat under the right political circumstances, competition will lead candidates to accentuate the differences between them. Its worth noting that the candidates who are finding it in their electoral interest to emphasize the differences with their opponents around embryonic stem cell research are largely moderate, pro-business Democrats running in traditionally Republican areas.