As passionate street protests erupt in parts of Tehran at what looks, prima facie, like an assisted landslide to re-elect Mahmoud Ahmadi-Nejad in a highly contested presidential vote, only one thing is clear: Iran's ruling theocrats are taking a huge gamble with the future of the Islamic Republic.

Domestically, they are patching up a dam that is starting to burst with pent-up desire for change. Abroad they are courting isolation: Barack Obama's hand, remember, was extended to those who would unclench their fists – not, on the face of it, what has just happened.

It was always the case that Mir-Hossein Moussavi, the former premier whose campaign realigned chastened reformists with pragmatic conservatives, and pulled hundreds of thousands of young and women supporters on to the streets, had a mountain to climb. The rural and urban poor and the regime's paramilitaries (the basij militia alone is about 12m strong) meant the president had vast reserves of pre-positioned support.