What if Al Qaeda had managed another attack on US soil on the same scale as the one on 9/11? Would it have the same cultural impact as 9/11 and how would it affect the United States after?
Scenario 1: July of 2004.
How would this have affected the 2004 elections? Would Bush still win due to sympathy votes, or would he lose because people feel his actions were not enough to keep the American People safe?
Scenario 2: Between February and May of 2009.
With this happening so early during Obama's first term, how would this change the following years? How would he handle such a disaster, and would he still win in 2012? Would 2016 be any different because of this?
In the first scenario, this splits the terrorism-issue voters. In the weeks leading up to the 2004 election, people saw the three biggest issues as terrorism, the economy, and the Iraq war. Terrorism voters trended toward Bush, economy voters trended toward Kerry, and Iraq voters trended 50-50. It ended up that the Iraq voters broke for Kerry but Bush won on the backs of "values voters."
Those "values voters" would overwhelmingly go with terrorism as their top issue, as would many economy and Iraq voters; however, many terrorism voters would be scared that Bush oversaw two terrorist attacks on American soil in four years, especially the two worst. People would abandon ship for Kerry, but that's probably the worst thing that could happen for the Democrats, as the recession is around the corner and much worse, especially with Katrina smacking New Orleans.
John McCain wins in 2008 and Obama ends up staying in the Senate, as does HRC. However, anemic recovery aided by two terms for the GOP (although no more terrorism; even the Boston Marathon bombing may be averted) puts Bernie Sanders in the White House in 2016 virtually unopposed in the primaries, whereas people see just about everyone on the GOP side as more of the same.
In scenario 2, it will change Obama a LOT. People will rally around him - he's inspiring and gives a great speech, and he will play the toughness card while his focus will be less on State actors and more on groups such as ISIS and al-Qaeda. He will capture bin Laden in short order and redirect military action to controlled strikes against groups rather than massive invasions.
The big thing that will really throw that strategy - which will work wonders for a couple of years - is the Arab Spring. It will be a lot harder to accomplish his goals when he has as much chaos on his hands as he does. He'll still get re-elected, but his second term won't be chocked full of successes and he may even end up in some kind of scandal from his handling of the Middle East.
His respect level will increase temporarily, though, and that likely means some gains for his domestic agenda. Obamacare becomes more robust, and maybe there's some action toward gun control. He may also push for a second stimulus package, reinvigorating the economy and giving a boost to whoever runs on the blue ticket in 2016 - perhaps even Biden.