BANJUL, Gambia — President Yahya Jammeh once predicted that his rule could last a billion years.

Now, the fate of his nation is hanging on one more anxiety-filled day.

After acknowledging defeat in an election last month, Mr. Jammeh abruptly changed his mind, refusing to step aside for the inauguration of the new president scheduled for Thursday and threatening to drag the nation into a bloody standoff.

Mr. Jammeh, who has long been criticized for human rights abuses and grandiose claims like being able to cure AIDS with little more than prayer and a banana, has insisted on a do-over election. He declared a state of emergency on Tuesday, warning the nation not to engage in any “acts of disobedience.”

West African nations are preparing to enter the country and force Mr. Jammeh’s ouster if he does not leave. In response, Mr. Jammeh has threatened that his own military is prepared to defend Gambia’s sovereignty.