As Atrios, Kevin, and others are arguing, Iran may well be the next Iraq, at least in a political sense. Given the flagging fortunes of the Republican majority, an unhinged, unleashed islamic Republic wielding nuclear weapons is the perfect excuse to refocus the election on national security. Perfect because they're scary, sure, but perfect because they're real, too. Ahmadinejad's ascension and Iran's admitted, verified race for nuclear weapons are both independently observable events that are rightfully heightening focus and fears on the country. Unlike Iraq, where Bush conjured an urgent, rapidly-massing threat out of whole cloth, circumstances in Iran are changing for the worse and paying close attention to their evolution makes perfect sense.
An invasion doesn't.
But, as Kevin says, Democrats are going to have to figure out a way to handle the issue, both because it'll prove politically salient and because, on its own terms, it's genuinely legitimate. And, granted, I don't know how to do that, but being proactive rather than reactive strikes me as a start. If Reid's offices aren't packed with exhausted national security aides drawing up a comprehensive plan to deal with Iran and the DNC's PR guys aren't booking prominent Democrats to blanket television and set the terms of the debate, we've got a problem. One way or the other, Iran is going to be an issue. And given that, Democrats need to step forward on it first so, in six months, they're not left playing catch-up to the hawks.
Update: Also, read Tim F.