Maybe Bush's democracy agenda will be so successful that foreign policy if off the table in 2008 or 2012. And Democrats can then swoop in with their unbeatable economic/cultural message. Fine. But the price of all that is that Republicans further enhance their long-standing image as the reliable foreign policy party. The fall of the Soviet Union did a good deal of enhancing in 1989; as did the liberation of Kuwait in 1991; as did, I think, some of Nixon's successes. These are all somewhat contingent events (i.e. Democrats could have accomplished similar things), but they helped build the Republican mystique. And eventually, foreign policy will come back to the fore in elections. It always has and it always will. But if Republicans and only Republicans can take credit for successes past (i.e. Bush's foreign policy, assuming it succeeds), they'll be instant winners at the polls once more.
Maybe. But probably not. The only way for a party to build that mystique is to have one of their members in the Oval Office during a time of national crisis. George W. Bush, for instance, was not considered a particularly good choice on foreign policy until that fateful day he spent reading "My Pet Goat". In fact, Gore led in foreign policy throughout the campaign, with the final poll showing that Americans trusted Al more than George, 50%-36%. And, if not for Gore's lackluster performance and the shenanigans in Florida, he would have occupied the White House on 9/11, and Democrats would have benefitted from one of their own proving himself a strong and capable commander-in-chief presiding during a national crisis.
In fact, and I hate to say this, Democrats were extraordinarily unlucky not to have a foreign policy crisis occur during Clinton's presidency. There was no Kuwait (Bosnia/Kosovo was a humanitarian crisis, which is a bit different), no Cuban Missile Crisis, no Cold War treaties, and certainly no 9/11. And that meant no opportunity to undo the party's lackluster image on national security. In fact, the only crisis a "recently" elected Democrat has presided over was the Iranian hostage crisis, which saw our arms tied and our rescue mission downed via mechanical malfunctions (and if you think Reagan could have done better on that one, please go read up on your Iran-Contra). Moreover, Democrats are less likely to respond to foreign provocation with invasions and overwhelming displays of force when unnecessary. There's no doubt in my mind that Bush 43 would have invaded Somalia in the aftermath of Mogadishu, accomplishing nothing but proving America's masculinity by sacrificing a few score of its young, and likely boosting his poll numbers in the process. We don't do that, and so we don't get to improve our public perception by implementing destructive and idiotic policies.
So the reason that it's important to take national security off the table and get a Democrat into the Oval Office is precisely so, if another crisis does hit, a member of our party is in place to deal with it, thus proving Democratic competence and strength. If Al Gore had been in office during 9/11, not only would he have sailed to reelection, but he'd likely be passing universal health care with the political capital he didn't squander on Iraq. And any future Democrat elected to the presidency, even if he gets there on domestic issues (as Bush did), will be in place to remake the image of his party in the event that a crisis does occur.
Preemptive Clarification: It should be obvious, but just to be clear, I'm discussing the politics of foreign policy crises here. Of course I'd prefer it if there never was another one. But the fact remains that such events have electoral ramifications and are often exploited with astounding cynicism as soon as the next election rolls around (see Cleland, Max). So not only would I rather a Democrat in office for policy reasons, but I'd much prefer to deny Republicans a club to beat their domestic opponents with. In any case, with massive suicide bombs erupting in Iraq and Lebanon returning to Syria's comfy embrace, I fear Matt's thesis is wrong anyway. For the moment, at least, it really does look as if freedom is on the retreat.