Mark Ribbing writes an excellent article entitled The 'Security Thing' in Opinion section of The Wall Street Journal of Wednesday, July 25, 2007. Unfortunately, the article is only available online to subscribers of the WSJ.
In the article, Democrat Ribbing makes the point that in perilous times, today's voters prefer a Republican in the White House. Since Vietnam, voters have only elected Democrats during periods when threat levels were low. He argues that in order to win in 2008, Democrats absolutely must prove to the voters that they are serious about terrorism and homeland security. He adds that talking about how quickly they can pull troops out of Iraq isn't the way to make this argument convincingly.
If you have access to Wednesday's WSJ, give it a look. I believe he is correct and, like him, I am far from sure that the Democrats can make this sale.