The group that gained more political participation when amendments were added to the Constitution is primarily African Americans. The most significant amendments related to their political participation are the 15th Amendment, which granted African American men the right to vote, and the 19th Amendment, which granted women the right to vote (though this primarily affected women, many African American women faced additional barriers).
Native Americans, Hispanic Americans, and Asian Americans also fought for political rights, but their gains were not directly tied to specific constitutional amendments in the same way that African Americans' rights were significantly impacted by those amendments. Many Native Americans were not granted U.S. citizenship until 1924, which limited their political participation even further.
Therefore, the best answer in the context of your question is African Americans.