Once an election is done, it is hard to undo.

That’s true in Iran, and it’s also true in the United States.

This is why it is important to get the rules by which elections are held right before elections are held.

For this reason, one of the essential components of the Voting Rights Act — arguably its most powerful tool for combating discrimination and disenfranchisement — has long been a requirement that officials get approval from the Department of Justice before they change the way in which elections are conducted.

Allow states, counties, municipalities or school districts in the 16 states that are wholly or partially with historic patterns of discrimination to opt out of the review, and they will be able to organize and hold elections that renew those patterns. That’s why the requirement has been referred to by law professors as “one of the crown jewels of the civil rights movement.”

Foes of the Voting Rights Act have long focused on weakening Section 5 of the act, the provision that requires election officials in the states covered by the act to obtain federal permission before making changes to voting procedures, moving polling-place locations, requiring so-called “citizenship checks” and redrawing voting district lines. They rightly argued that to do so would remove the teeth from the measure that has long been disdained by southerners pining for the days before what former Senate Majority Leader Trent Lott referred to as “all the laws of Washington” changed the way things were done in Dixie.

On Monday, the Supreme Court tarnished the crown jewel, giving state and local officials new flexibility to “opt out” of the requirement that they obtain permission when changing election rules. The court ruling does not invalidate the Voting Rights Act — as some had feared — but it does undermine it.

The court, with only one justice (Clarence Thomas) in partial dissent, said that the Northwest Austin Municipal Utility District No. 1 in Austin, Texas, can avoid the advance approval requirement.

The ruling is being interpreted as a signal all local jurisdictions in a Voting Rights Act state can at least apply for what is referred to as “a statutory bailout.”

That was a reversal of a lower federal court that had preserved the Voting Rights Act as it was intended to operate.

That’s a dangerous move, say civil rights supporters.

As Georgia Congressman John Lewis, who has watched the court’s deliberations closely, says, “No one can deny the fact we’ve made progress. But that’s not the question. That’s not the issue. The issue is we need this tool to guard against the possibility of reverting back to our dark past.”

Lewis is right. Invalidating the Voting Rights Act would be a shock to the body politic. But dismantling the measure tooth by tooth should still be recognized for what it is: a judicial assault on history, and on the future.

The Voting Rights Act is still on the books — despite evidence from recent hearings that Chief Justice John Roberts and some of his conservative activist colleagues would like to do away with it. Voters can still sue under its provisions when they believe they are victims of discrimination. Unfortunately, notes Laughlin McDonald, who directs the ACLU’s voting rights project, few plaintiffs will have the financial resources to pursue these complex cases.

So the high court has taken a big whack at the Voting Rights Act.

Now it falls to the Obama administration’s Department of Justice — which has sent good signals regarding its commitment to enforcing voting rights protections — and the Congress to put the teeth back in the act.

Senate Judiciary Committee chairman Patrick Leahy, D-Vermont, has warned that any attempt by the court to strike down the Voting Rights Act “would be conservative activism pure and simple.”

The same goes for pulling the act apart tooth by tooth.