"Regarding deference, not that long ago, the Justices believed Congress held something close to plenary power when it crafted remedies addressing racial discrimination in voting. In case after case, the Justices made clear that they would not second-guess congressional judgments on the subject. Even as the Justices began looking more rigorously at particular types of congressional remedial action elsewhere, they repeatedly distinguished the invalidated laws from the VRA and celebrated provisions like preclearance as paradigmatic examples of permissible congressional action...
"The decision significantly diminishes Congress’s ability to craft future remedies for racial discrimination in voting and beyond. Indeed, after today, an administrative agency acting within the sphere of its expertise enjoys more discretion than does Congress when acting in the realm in which its power was once viewed to be at its apogee.
"At oral argument last winter, Justice Kagan bristled at the notion that the Court, rather than Congress, was the proper institution to decide when remedial action in this realm was needed. Justice Scalia was nevertheless convinced that “[t]his is not the kind of question you can leave to Congress.” Today’s decision makes clear that a majority of the Court shares this view. Earl Warren would have been astounded. William Rehnquist, too."
http://www.scotusblog.com/2013/06/how-big-is-shelby-county/#...