In 1964 a wave of violent demonstrations over voting rights in the South, culminating in an attack by state troopers on peaceful marchers at Selma, Alabama, had convinced President Johnson that voting reform was long overdue. Congress passed the Voting Rights Act on 6 August 1965. It used the 14th and 15th Amendments to the US Constitution to pass a law that African Americans should be allowed to exercise their legal right to vote, without obstruction at state or local level. Discriminatory practices that were common in the South to hinder black voter registration, for example literacy tests, were outlawed, as well as harassment and intimidation. Federal examiners were enlisted and despatched to the South to conduct voter registration (August 1965–67) and to ensure the Act was fully honoured. By the end of 1965 250,000 black voters had been registered, one-third by Federal examiners. The Act has been called the most successful piece of civil rights legislation ever passed by the US government.