She opposed the 15th amendment

In reaction to the proposed 15th Amendment, Cady Stanton and Anthony also established the National Woman Suffrage Association in 1869. Ginzberg asserts that after the Civil War, when Congress discussed liberated slaves' voting rights, feminists were presented with a decision. Ginzberg told NPR that there was a debate among abolitionists—among whom Stanton counted herself—over whether to support a 15th Amendment giving black males the right to vote or to hold out for a suffrage amendment giving all adult Americans the right to vote.

"Stanton and her friend Susan B. Anthony stood on what they believed to be the highest moral ground by demanding universal human rights for all and—historians have debated this ever since—not being willing to sacrifice women's rights for the politically expedient challenge of gaining rights for black men," according to the article. The 15th Amendment, giving men the right to vote regardless of “race, color, or previous condition of servitude,” was ratified in 1870. Women did not end up achieving the franchise until 1920.