Obama's decision to pick a running mate who voted for the Iraq war resolution shows how dramatically the nature of his candidacy has changed over the past year. There are many reasons why people love Obama, but his campaign was forged on the anvil of anti-war fury. Had Hillary Clinton opposed the war in 2002, Obama probably would not have run at all, and she would most likely be the Democratic nominee today.
But this campaign is no longer primarily about Iraq. The drop in violence and US casualties there have pushed the war off the front pages. It seems possible that voters consider the original decision to go to war a distant and well-litigated memory. And while Obama appears to be winning the argument about withdrawal, now that the Bush administration is committing to ("aspirational") timelines, he's not reaping great rewards with voters. As I've argued before, I think the sense that no one wants America to stay in Iraq for 100 years anymore may actually be helpful to John McCain, since it deflates one of Obama's most winning issue contrasts.
The Russia-Georgia conflict aside, this campaign is increasingly about economic pain. In that sense it has become much more like the 1992 election--"it's the economy, stupid." Obama still needs to reassure voters that he can manage an international crisis, and Biden helps him do that. But one of the real lessons here is that Iraq is simply not the defining litmus test it was several months ago.