What would happen if there were a new terrorist attack inside the United States on 11 September 2004? How would it affect the presidential election campaign? The conventional wisdom is that Americans - their patriotic defiance aroused - would rally to President George W Bush and make him an all but certain winner in November.
But consider the differences between the context of the original
9/11 and that of any attack which might occur this autumn. In 2001, the public reaction was one of disbelief and incomprehension. Many Americans realised for the first time that large-scale terrorist attacks on US soil were not only conceivable; they were, perhaps, inevitable. A majority focused for the first time on the threat from al-Qaeda, on the Taliban and on the extent to which Saudis were involved in terrorism.
This time, the public response would move much more quickly from shock to anger; debate over how America should respond would begin immediately. Yet it is difficult to imagine how the Bush administration could focus its response on an external enemy. Should the US send 50,000 troops to the Afghan-Pakistani border to intensify the hunt for Osama Bin Laden and "step up" efforts to attack the heart of al-Qaeda? Many would wonder if that wasn't what the administration pledged to do after the attacks three years ago. The president would face intensified criticism from those who have argued all along that Iraq was a distraction from "the real war on terror".
And what if a significant number of the terrorists responsible for the pre-election attack were again Saudis? The Bush administration could hardly take military action against the Saudi government at a time when crude-oil prices are already more than $45 a barrel and global supply is stretched to the limit. While the Saudi royal family might support a co-ordinated attack against terrorist camps, real or imagined, near the Yemeni border - where recent searches for al-Qaeda have concentrated - that would seem like a trivial, insufficient retaliation for an attack on the US mainland. Remember how the Republicans criticised Bill Clinton's administration for ineffectually "bouncing the rubble" in Afghanistan after the al-Qaeda attacks on the US embassies in Kenya and Tanzania in the 1990s.
So what kind of response might be credible? Washington's concerns about Iran are rising. The 9/11 commission report noted evidence of co-operation between Iran and al-Qaeda operatives, if not direct Iranian advance knowledge of the 9/11 hijacking plot. Over the past few weeks, US officials have been more explicit, too, in declaring Iran's nuclear programme "unacceptable". However, in the absence of an official Iranian claim of responsibility for this hypothetical terrorist attack, the domestic opposition to such a war and the international outcry it would provoke would make quick action against Iran unthinkable.
In short, a decisive response from Bush could not be external. It would have to be domestic. Instead of Donald Rumsfeld, the defence secretary, leading a war effort abroad, Tom Ridge, the homeland security secretary, and John Ashcroft, the attorney general, would pursue an anti-terror campaign at home. Forced to use legal tools more controversial than those provided by the Patriot Act, Americans would experience stepped-up domestic surveillance and border controls, much tighter security in public places and the detention of a large number of suspects. Many Americans would undoubtedly support such moves. But concern for civil liberties and personal freedom would ensure that the government would have nowhere near the public support it enjoyed for the invasion of Afghanistan.
Far from bolstering a Bush candidacy, the polarising pressure of elections would nullify the rally-around-the-flag effect. Perhaps if an attack occurred in the final two or three days before balloting - as it did in Madrid - the national conversation about possible responses might not come into play, and Bush could receive the political benefit of undiluted anger at terrorists. In any other circumstance, however, an attack would benefit Senator John Kerry.
Ian Bremmer is president of Eurasia Group and senior fellow at the World Policy Institute