Two weeks ago Egypt's temporary leadership lifted a decades-old "state of emergency". Today martial law is back, in anticipation of a court ruling that could throw the hotly disputed presidential election wide open again and bring demonstrators back to the streets.

Later today, Egypt's supreme constitutional court may disqualify one of the two candidates in this weekend's run-off vote. Ahmed Shafiq was prime minister under jailed former President Hosni Mubarak: a new law bars senior members of the old regime from standing for office under. But that law that may yet be deemed unconstitutional.

Meanwhile, the court has to decide whether Shafiq's candidature was valid, and may rule that the first round vote has to be restaged in its entirety – hence the army's nervousness and the return of martial law.

Back to the bad old days?

Egyptians, including many of those who demonstrated for democracy in Tahrir Square last year, may welcome a rerun. Shafiq's removal would please those who want a change from the old, corrupt regime. The other candidate, Mohammed Morsi, head of the Muslim Brotherhood’s political arm, is far ahead in the polls. He and Shafiq are seen as representative of two extremes of Egyptian politics – neither of them hold any banner for open, secular democracy.

But in the fevered atmosphere of Egypt at the moment, any threat to the democratic process brings with it all sorts of fears. And, despite agreement on who will write the new democratic constitution just last week, the new martial law gives back sweeping powers to the military – including the right to arrest civilians for "resisting authorities", and to try them in army courts.

Read more: Excellent analysis of the strength and popularity of the Muslim Brotherhood, from the Foreign Policy research institute.