U.S. Attorney General Eric H. Holder Jr., appearing at a Martin Luther King Jr. holiday rally in South Carolina, warned Monday that voting rights laws are still at risk and said aggressive enforcement of those laws is “a moral imperative.”

Weeks after his Justice Department blocked a South Carolina voter-identification law it said would make it harder for tens of thousands of voters, mostly minorities, to cast a ballot, Holder said the principle of electoral equality was still endangered.

“The reality is that — in jurisdictions across the country — both overt and subtle forms of discrimination remain all too common,” Holder told hundreds of people attending an annual rally to honor the slain civil rights leader, on the steps of the South Carolina State House.

“Protecting the right to vote, ensuring meaningful access and combating discrimination must be viewed not only as a legal issue, but as a moral imperative,” Holder said. “Ensuring that every eligible citizen has the right to vote must become our common cause.”

The South Carolina law required voters to show a state-issued photo identification card to cast a ballot in an election. Republican supporters said it would prevent voter fraud, but Democratic critics argued that it would make it harder for those without driver’s licenses, many of them poor and black, to cast a ballot.

Justice blocked the law, noting that just over a third of the state’s minorities who are registered voters did not have a driver’s license. The state plans to fight the ruling in court.

South Carolina is one of six Republican-led states that tightened their laws last year to require a photo ID. Two other Republican-led states have similar laws in place, while 23 other states require voters to produce some form of identification.

Under the landmark 1965 Voting Rights Act, South Carolina is one of 16 largely Southern states that must seek approval from Justice or the federal courts for changes made to state voting laws and boundaries for voting districts.

“This keystone of our voting rights laws is now being challenged as unconstitutional by several jurisdictions,” Holder said, adding there was still work to be done to ensure voter equality.

Holder was invited to the annual rally to honor King by the state chapter of the NAACP.

South Carolina holds its Republican presidential primary on Saturday. The Republican candidates have criticized Justice’s ruling as an example of Washington’s bureaucratic intrusion on state rights under President Obama.

— Reuters