Against the opposition of their four colleagues, five right-wing Supreme Court justices have now guaranteed that big corporations can spend unlimited funds on political advertising in any political election. In an opinion written by Justice Anthony Kennedy and joined by Chief Justice John Roberts and Justices Samuel Alito, Antonin Scalia, and Clarence Thomas, the Court overruled established precedents and declared dozens of national and state statutes unconstitutional, including the McCain-Feingold Act, which forbade corporate or union television advertising that endorses or opposes a particular candidate.

This appalling decision, in Citizens United v. Federal Election Commission, was quickly denounced by President Obama as “devastating”; he said that it “strikes at our democracy itself.” In his State of the Union speech of January 27, he said, “Last week, the Supreme Court reversed a century of law that I believe will open the floodgates for special interests—including foreign corporations—to spend without limit in our elections.” He is right: the decision will further weaken the quality and fairness of our politics.

The Court has given lobbyists, already much too powerful, a nuclear weapon. Some lawyers have predicted that corporations will not take full advantage of it: they will want to keep their money for their business. But that would still permit carefully targeted threats. What legislator tempted to vote for health care reform or Obama’s banking reorganization would be indifferent to the prospect that his reelection campaign could be swamped in a tsunami of expensive negative advertising? How many corporations fearful of environmental or product liability litigation would pass up the chance to tip the balance in a state judicial election?

On the most generous understanding the decision displays the five justices’ instinctive favoritism of corporate interests. But some commentators, including The New York Times, have suggested a darker interpretation. The five justices may have assumed that allowing corporations to spend freely against candidates would favor Republicans; perhaps they overruled long-established laws and precedents out of partisan zeal. If so, their decision would stand beside the Court’s 2000 decision in Bush v. Gore as an unprincipled political act with terrible consequences for the nation.

We should notice not just the bad consequences of the decision, however, but the poor quality of the arguments Justice Kennedy offered to defend it. The conservative justices savaged canons of judicial restraint they themselves have long praised. Chief Justice Roberts takes every opportunity to repeat what he said, under oath, in his Senate nomination hearings: that the Supreme Court should avoid declaring any statute unconstitutional unless it cannot decide the case before it in any other way. Now consider how shamelessly he and the other justices who voted with the majority ignored that constraint in their haste to declare…