That picture above is, according to Gallup, the underlying issue preferences of the American public. It's interesting to note that even in these polarized times, there appear to be a substantial number of people who are at least in some sense torn between McCain and Obama preferring one on some issues and the other on some other issues.
You also see that national security has turned into a substantial disadvantage for Obama -- he's tied on the question of Iraq and losing badly on the question of terrorism. But he's in the lead because these are, at the moment, relatively low-salience issues compared to people's economic grievances. This leaves Obama with a choice of campaign strategies, he can try to emphasize the issues he's winning on, hoping to maintain the current low salience of security, or he can attempt to shore up his weaknesses by talking about national security and trying to persuade people that his vision of an aggressive, but focused and disciplined, full-spectrum campaign against al-Qaeda is the best way to keep the country safe.
The conventional Democratic strategy would be to try to duck the debate and hope the economy will carry him through. That kind of thinking is, however, one of the reasons Democrats have had their Heads in the Sand for many years. It's relatively likely that events in the world will lead to a renewed focus on national security at some point in the coming months, and it's also relatively easy for the McCain campaign to change subjects in this direction at a time of their choosing since security issues are, by their nature, visceral and frightening.
At the same time, McCain is the heir to eight years of failed policymaking from the Bush administration and Obama has a very solid case to make that he can do better. But will he make it aggressively?