In a rare Saturday morning order, the U.S. Supreme Court said it will allow Texas to enforce a voter identification law in next month's midterm elections. The law’s opponents say that it will restrict the voting rights of hundreds of thousands of voters, the overwhelming majority of whom are poor and minority citizens.

A majority of justices rejected an emergency request from the Department of Justice and civil-rights groups to keep the state from enacting a law that requires citizens to produce prescribed forms of photo identification before they could cast a ballot, while three justices—Ruth Bader Ginsburg, Sonia Sotomayor and Elena Kagan—dissented.

"The greatest threat to public confidence in elections in this case is the prospect of enforcing a purposefully discriminatory law, one that likely imposes an unconstitutional poll tax and risks denying the right to vote to hundreds of thousands of eligible voters," wrote Ginsburg.

Last week, a federal judge struck down the restrictive law, saying that roughly 600,000 voters, many of them black or Latino, could be turned away at the polls because the were lacking the sufficient identification. In a 143-page opinion following a two-week trial, U.S. District Judge Nelva Gonzales Ramos called the law an "unconstitutional burden on the right to vote," finding that the Republican-led Texas state legislature intentionally discriminated against minority voters. However, a federal appeals court on Tuesday put that ruling on hold.

"The last thing the Supreme Court ought to do in response to a district court finding of intentional voting discrimination is to accept Texas’ excuses for using the discriminatory law in a general election," said Bob Kengle, co-director of the Voting Rights Project at the Lawyers’ Committee for Civil Rights Under Law, in a statement.

Under Chief Justice John Roberts, the Supreme Court has repeatedly altered state and federal law governing who can vote and when.  In June 2013, the Court struck down a key part of the Voting Rights Act that had prohibited Texas and eight other states with histories of racial discrimination from altering election laws without federal approval. Texas quickly passed the tougher restrictions that the court issued an order about today.

One of the key questions about the Texas law is whether being made to produce ID to vote will affect turnout at all. (In 2011, South Carolina’s governor Nikki Haley made an offer to citizens who felt burdened by a similar law: "Find me those people that think that this is invading their rights, and I will go take them to the DMV myself," she said.) But the largest non-partisan federal study conducted on the issue—released just last week—found that, in at least two other states, yes, voter ID laws really do depress the number of people who vote. The Government Accountability Office reported that in the 2012 election, some 100,000 fewer people voted in Kentucky and Tennessee because of those state's voter ID laws—and a disproportionate number of those non-voters were blacks and young people.

Over the past month, the Supreme Court has had its say in other elections by allowing Ohio to curtail early voting, blocking Wisconsin from implementing a new voter ID law and letting North Carolina abolish same-day voter registration.