After the full rights of citizenship had been by Amendment XIV conferred upon negroes who came within the description of citizenship enunciated in that amendment, it was thought desirable that such persons should not be excluded on account of their race or color from participation in affairs of government under the same conditions imposed as to white persons; and accordingly Amendment XV was incorporated into the federal constitution in these words: "The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any state on account of race, color, or previous condition of servitude."