I was recently giving some lectures on the topic of presidential elections at a family summer camp in the Sierras (great gig, by the way). I was trying to give the audience a sense of what actually affected votes (i.e.: income growth in the election year, wars) and what didn't (i.e.: pretty much everything else). Several audience members, however, wanted to know why I hadn't mentioned issues. Surely the candidates' stances on things like abortion, birth control, gun access, gay rights, etc. affect people's votes, right?
To be sure, the candidates spend a good deal of staff time developing public positions on a broad array of issues. (Obama's issue positions are here, Romney's are here.) But do those things actually affect votes? The basic answer is no. But it's not that issues don't matter, it's just that they're already rolled into people's impressions of the major parties.
Suffice it to say that the vast majority of Americans will never visit the above web pages and will never comb through newspaper coverage to find out what Obama and Romney feel about particular issues. Overwhelmingly, they will rely upon the "D" or "R" label that appears next to the candidate's name and just infer the candidate's beliefs from that. And that's actually pretty reliable! Especially in a period of strong party polarization, the candidate's party affiliation tells you just about everything you need to know about his or her issue stances. Parties very rarely nominate someone who's less than doctrinaire about major party commitments, whether they're running for president or governor or just about any other office.
Now, once in a while, there are exceptions. Barry Goldwater was probably more conservative than the average Republican nominee in 1964 on a few key issues (notably, the use of force abroad), and that may well have cost his ticket votes. George McGovern was probably more liberal than the average Democratic nominee in 1972, and that probably cost his ticket some votes, too. And issue stances undoubtedly matter in primaries. That's how party leaders and activists develop a sense of which candidates would be good for the party and which wouldn't. Those issue stances on the candidates' websites in the general election mainly serve to signal to the candidates' supporters from the primaries that the candidates haven't betrayed them.
But it's rare that those issue stances communicate any information beyond "traditional Democrat" or "traditional Republican." Issues matter, but only because the parties already care about them.