TUNIS — Tunisia’s political crisis entered a new phase Sunday with an announcement that Cabinet ministers of the president’s own party are quitting the governing coalition, which could force the ruling Islamists to compromise with the opposition.

Veteran observers of Tunisia’s politics caution that the nation’s well-earned reputation as a stable bastion of moderation risks being put to the test, if the ruling Ennahda Party of moderate Islamists mishandles its response to the assassination Wednesday of opposition politician Chokri Belaid.