Which law or amendment to the constitution had the greatest impact on expanding voting rights between 1865 to today?

15th Amendment.

Explanation:

The 15th Amendment of the Constitution of America allowed all men to vote without racism and discrimination. The law after the Civil War granted the rights to the African Americans (now freed) to vote. Before ratifying the amendment, only white men in America were allowed to vote and elect the government. The 15th Amendment was rectified during the Reconstruction era along with other Amendments like the 13th and 14th.

fourteenth amendment

no every country is held to the same degree as others just some member countries such as the usa and germany are more economically sound so the need to show there support to all others