How did south carolina v katzenbach and shelby v holder affect the voting rights act of 1965

In South Carolina v. Katzenbach ( and similarly and much more recently, in 2013, in Shelby County v. Holder) South Carolina filed a complaint  with the Supreme Court challenging the Voting Rights Act as unconstitutionalon respect to state´s rights. Many outhern states follow suit. This provoked the Supreme Court to pass judgement before the elections of 1966, upholding the Voting Rights Act and thus enabling almost a million of African Americans to register to vote.

it reintroduced the two-party system.