The Senatus Consultum Ultimum is identical to the Senatus Consultum de Republica Defendenda.

The Senatus Consultum de Republica Defendenda was first passed by the Senate to deal with mass protest of Gaius Gracchus on the Aventine Hill in 121 BC; it avoided the need to elect a dictator rei gerendae causa. Consul Lucius Opimius understood the Senatus Consultum de Republica Defendenda as an order to suppress their activites by any means necessary, including force. Thereafter Opimius formed an army of Senators and their clients, and confronted Gracchus and his supporters in a pitched battle within the walls of Roma.

Thereafter the Senatus Consultum de Republica Defendenda was understood to mean that it declared martial law and empowered the consuls to "take care that the Republic should take no harm". During the Cataline Conspiracy Marcus Tullius Cicero found it expedient to refer to it every other sentence and found the full title cumbersome; thus he began to call it the Senatus Consultum Ultimum.