Final answer:
The 24th Amendment made it illegal to charge a poll tax to vote, specifically targeting practices used in the South to disenfranchise African American voters by removing a financial barrier to voting in federal elections.
Step-by-step explanation:
The amendment that made it illegal to charge a poll tax to vote is the 24th Amendment. The 24th Amendment to the U.S. Constitution outlawed the practice of charging a poll tax in national elections. Poll taxes were a method predominantly used in the Southern states to disenfranchise African American voters and others who were economically disadvantaged. Although the 15th Amendment guaranteed the right to vote regardless of race, color, or previous condition of servitude, states found ways around this, such as the imposition of poll taxes, which served as a barrier to voting for many.
The passage of the 24th Amendment in 1964 specifically targeted this practice and prohibited the requirement of a tax payment as a condition for voting in federal elections. This amendment was a significant stride in ensuring the voting rights of all citizens, as it removed a financial barrier that had unfairly restricted access to the ballot. Further enforcing this right, the Supreme Court, in Harper v. Virginia Board of Elections (1966), ruled that poll taxes were unconstitutional at any level of elections, extending the impact of the 24th Amendment to state and local elections as well, thus fortifying the democratic process and marking a key victory in the Civil Rights Movement.