With some US embassies now evacuating non-essential staff, and platoons of Marines sent to particular hotspots, the anti-American demonstrations provoked by an obscure anti-Islamic film trailer are not just an indictment of divisive religious extremism and the politics of the mob. They might also prove a pivotal moment for the US presidential elections. And it is President Obama whose position is most exposed.

Quite apart from the human tragedy, the killing of Chris Stevens, the US ambassador to Libya, in Benghazi last week is of huge symbolic value. At first glance, the President might appear to have come out on top. Mitt Romney's too-quick, overly political response handed Mr Obama the opportunity to accuse his Republican challenger of a tendency to "shoot first and aim later" – not a good qualification for a prospective commander-in-chief. By catapulting overseas concerns to the top of the agenda, the disturbances also emphasise the lack of foreign policy details from Mr Romney so far.

But it would be unwise to overlook the extent to which protecting American lives – both at home and abroad – is viewed by many US voters as the primary gauge of presidential competence. Just as the Iranian hostage crisis proved fatal to Jimmy Carter's hopes of re-election, so too George W Bush's no-holds-barred response to the 9/11 terrorist attacks (which were too early in his presidency to be considered his fault) went a long way to guaranteeing a second term. Against such a standard, the past week's events play very badly indeed for the incumbent.

As a Democrat, Mr Obama always had much to prove on issues of security. In fact, he has handled the Arab Spring well, carefully standing back from overt influence while working hard to build relations with new leaders. But the spectacle of embassies under siege and diplomats killed – for the first time in decades – threatens to see such deftness characterised as weakness, perhaps even cancelling out the bounce from the killing of Osama bin Laden.

For much of middle America, realpolitik and the long diplomatic game are as nothing compared with the brute fact of their compatriots' safety. Mr Obama has some explaining to do.