I just posted something in the Donald Trump thread about the risk of terrorist activity in the time leading up the the US elections.
A terrorist attack would play into the hands of Trump, with his anti-Muslim, anti-immigrant rhetoric. It would garner support for his extreme rantings, increase the suspicion and hatred towards foreigners (especially of a middle-eastern extraction) and give a massive boost to his chance of becoming president.
Yet Trump becoming president could be exactly what the likes of ISIS would want. He would create the sort of divisive environment in which they thrive, giving them more opportunities to manipulate those disaffected by the feeling of alienation and increasing the opportunities to recruit more followers likely to carry out further atrocities. It could become a (very) vicious circle.