Three years ago, the Supreme Court declared the preclearance mandate unconstitutional, saying the blatant discrimination it was meant to prevent was largely a thing of the past.

But since the Supreme Court’s 5-to-4 ruling in the voting-rights case, Shelby County v. Holder, critics argue, the blatant efforts to keep minorities from voting have been supplanted by a blizzard of more subtle changes. Most conspicuous have been state efforts like voter ID laws or cutbacks in early voting periods, which critics say disproportionately affect minorities and the poor. Less apparent, but often just as contentious, have been numerous voting changes enacted in counties and towns across the South and elsewhere around the country.

They appear as Republican legislatures and election officials in the South and elsewhere have imposed statewide restrictions on voting that could depress turnout by minorities and other Democrat-leaning groups in a crucial presidential election year. Georgia and North Carolina, two states whose campaigns against so-called voter fraud have been cast by critics as aimed at black voters, could both be contested states in autumn’s presidential election.

Kristen Clarke, the president of the Lawyers’ Committee for Civil Rights Under Law, a leading voting-rights advocacy group, said that before the Supreme Court’s Shelby County ruling, discriminatory laws and procedures had been blocked by the preclearance provisions.

Now, she said, “We’re seeing widespread proliferation of these laws. And we are left only with the ability to mount slow, costly case-by-case challenges” to their legality.

Conservative critics of the Voting Rights Act say that is as it should be — that the federal government has no business usurping the role of elections monitor that citizen advocates have long and effectively played in other states. “Now every jurisdiction in the country must be treated equally in our courts when election issues are at stake,” said Edward Blum, the director of the Project on Fair Representation, a nonprofit legal program.