Final answer:
The issue that almost led to the failure of the Declaration of Independence was disagreements over slavery, specifically opposition from Southern colonies to language condemning the slave trade. The unifying cause for the colonies, however, was 'taxation without representation', which led to the Declaration of Independence.
Step-by-step explanation:
The issue that almost caused the Declaration of Independence to fail was disagreements over slavery. During debates at the Continental Congress, there was significant contention over the institution of slavery. Among the original drafts of the Declaration, there was a passage that condemned the enslavement of African people and the transatlantic slave trade, which was introduced by Thomas Jefferson. However, this clause was removed because of the objections from delegates representing Southern colonies, where slavery was integral to their economic systems. This compromise was critical to ensure the support of all thirteen colonies. The importance of rejecting 'taxation without representation' was a central theme leading up to the American Revolution. The colonists' resentment towards British taxes without having their own representatives in the British Parliament, represented by policies such as the Stamp Act, played a critical role in rallying the colonies towards independence. Eventually, the formation of a unified front against these British impositions led to the calling of the First Continental Congress, where colonists began to articulate their rights and grievances, ultimately concluding with the Declaration of Independence.