CAIRO — The Muslim Brotherhood, among the most powerful forces in Egypt, is facing perhaps the worst crisis in its 80-year history. Its members have been gunned down in the streets. Its new headquarters have been ransacked and burned, its political leader, President Mohamed Morsi, abandoned, threatened and isolated by old foes and recent allies.

It is a steep fall for the pre-eminent Islamist movement in the region, and especially surprising for a group that was elected just one year ago. Its critics say the Brotherhood remains stuck in old divisions, pitting Islamists against the military, and has failed to heed the demands of ordinary citizens.

“I think this is an existential crisis, and it’s much more serious than what they were subjected to by Nasser or Mubarak,” said Khaled Fahmy, a historian at the American University in Cairo, referring to the governments of Gamal Abdel Nasser and Hosni Mubarak, the autocrat deposed in 2011. “The Egyptian people are increasingly saying it is not about Islam versus secularism,” Mr. Fahmy said. “It is about Egypt versus a clique.”

But Mr. Morsi and the Brotherhood have made it clear they will not back down. After days of restraint, the movement’s members are fighting bloody street battles, convinced that hard-fought victories are being unfairly stripped away. Members have marched in the streets carrying death shrouds, and a senior leader urged members “to seek martyrdom” in the battle against “a military coup.”