The addition of new amendments to the Constitution has taken place slowly over many decades.
Historically, the U.S. has seen a gradual expansion of voting rights through various amendments and legislative actions. Key amendments include the 15th Amendment (1870), which prohibits denying the right to vote based on race; the 19th Amendment (1920), granting women's suffrage; the 24th Amendment (1964), eliminating poll taxes in federal elections; and the 26th Amendment (1971), lowering the voting age to 18. Each of these amendments reflects a significant and often contentious struggle for expanded enfranchisement, illustrating a slow and evolving process rather than rapid changes or a concentration solely in the 20th century.