The 16th amendment in 1913 permits Congress to lay and collect taxes on incomes, from whatever source derived, without apportioning it among the states or basing it on the US Census. Ratification was completed on February 3, 1913. Before that customs duties and excise taxes were the primary sources of federal revenue. Answer: The ratification of the 16th amendment.
The compaign methods from that year was also noteworthy for the intense personal attacks widely employed by the supporters of both candidates