EVER SINCE the Supreme Court gutted a key section of the 1965 Voting Rights Act, Attorney General Eric H. Holder’s Justice Department has been trying to patch it, using the sections of the law that the court left in place to reconstitute the checks on discrimination that had existed for decades. The Justice Department’s latest move, involving a challenge to odious new voting restrictions in North Carolina, demonstrates that Mr. Holder is committed to the effort. It also demonstrates why Congress, not the Obama administration, should be the branch of government offering the primary response to the court’s ruling.

Before the decision, the law obliged certain parts of the country with a history of racial discrimination to clear changes to electoral rules with the Justice Department or federal judges. Dozens of counties in North Carolina, for example, had to get approval before, say, adjusting electoral district lines, ensuring that reforms did not have the effect of limiting minorities’ access to the franchise. The high court did not strike down the concept of pre-clearance, but it did throw out the formula that determined which places had to submit to the rule.

Immediately after the court’s decision, states such as North Carolina approved indefensible voting policies that will make it harder for many people to cast ballots, with disproportionate effects on minority voters. North Carolina lawmakers, for example, cut the number of days voters would have to cast early ballots, eliminating a full week from the early-voting schedule. They also banned voters who show up to the wrong polling place from casting provisional ballots. They added a requirement that would-be voters present one of a few forms of identification, without much recourse for people who lack the documents. All of these, the Justice Department argues in a complaint it filed last week, disproportionately impact African Americans in North Carolina.

To fully prevail, though, Obama administration lawyers don’t simply need to show that North Carolina’s law has the effect of disproportionately limiting African Americans’ access to the ballot box; they also must demonstrate that state lawmakers intended to discriminate against black North Carolinians. That’s because the Justice Department isn’t simply asking a judge to throw out the law — it’s also arguing that North Carolina should be subject to pre-clearance again, which federal courts can still order on a case-by-case basis. Proving intent is difficult. Many of the law’s reforms have little good justification, but Justice will have a hard time showing that the motives behind them were racial, not simply partisan.

With a series of wins in cases such as North Carolina’s, the Justice Department could reestablish the pre-clearance requirement in many places where it used to apply. The easier and fairer way to revive pre-clearance, however, would be for Congress to rewrite the formula for which places should be covered. The Supreme Court left lawmakers that latitude, and large bipartisan majorities in Congress historically have supported pre-clearance. If lawmakers want to get back to doing something productive, resuscitating the Voting Rights Act would be a good place to start.