In particular, Steve has theorized that Johnson might impact the race with respect to the issue of marriage apartheid. Here's one of Steve's pieces on that, which links to another.
I'm personally more skeptical than Steve as to whether or not Johnson will have any great effect at all on the outcome (aside from possibly determining the disposition of New Mexico's five electoral votes -- he's that state's former two-term governor).
BUT! I think a lot of people are thinking about this in the wrong way. To steal one of Steve's quotes from a spokesperson for the American Anti-Family Association ... er, "National Organization for [sic] Marriage:
If Obama were to embrace same-sex marriage, he'd be on the wrong side of majorities in [several battleground] states.
And that's true, but it's nowhere near all the math.
First of all, it's a reasonable bet that most voters who are very much against same-sex marriage, and consider it an important issue, are already just not available to Obama under any circumstances. He loses nothing by further pissing them off.
Secondly, by getting fully and unambiguously on the right side of the issue (which, the news broke as I started this post, he just did), he energizes otherwise lukewarm supporters who care about the issue and didn't like seeing him remain on the fence.
Where does Johnson come in? Well, Johnson's been a supporter of marriage freedom for a long time, and has recently publicly reiterated his support for it from the bully pulpit of his presidential candidacy.
Had he not done so, Obama might have been able to remain on the fence. After all, it's not like Romney was an alternative for those "I support Obama but wish he'd come out for marriage freedom" voters.
But Johnson is right on that issue and Obama wasn't. And Johnson excels Obama by far on the ACLU's civil liberties canvass, too.
And the key here is that we're talking about battleground states, where a percentage point or two may be the difference between getting those electoral votes and not getting them.
A pro-marriage-freedom Democrat, especially one who also cares about other civil liberties issues just might be disappointed by Obama's record, and tempted by Johnson's.
And it just might only take a few of those voters to swing a close election.
So yeah, I think we can at least prospectively and partially credit Johnson with bringing Obama down off the fence, and on the right side of it, even.
And, well, it sucks to be Mitt right now. He's painted himself into an anti-family, pro-torture corner on marriage and civil liberties. He doesn't have any fences he can climb down from. His only hope with regard to pro-marriage-freedom and pro-civil-liberties Republicans is that he can satisfy them enough on other issues, or scare them badly enough about Obama on those other issues, to hold on to their votes. Too bad for him that Johnson's better than he is on foreign policy, immigration policy and economic policy, too.