It’s up to the Federal Election Commission or Congress to decide what the Supreme Court’s latest campaign finance decision means in practice. But we fear that both will punt on their responsibilities and allow corporations and unions back into the business of multimillion-dollar electioneering in the guise of issue advertising.

In 1907, Congress barred corporations from contributing to political campaigns. It applied the ban to unions in 1947 and extended the prohibition to expenditures as well as contributions.

The 1947 Taft-Hartley Act allowed both unions and corporations to form separate political action committees to give their employees and members the opportunity to express themselves politically. What was banned was the use of corporate or union treasuries for electioneering.

This formula has always struck us as the correct one, but corporations and unions have been finding channels around it for decades — most recently in the explosion of “soft money” contributions to party committees in 1996 and sham “issue ads” condemning the vices of various candidates at election time.

Congress banned both soft money and pre-election issue ads that mention candidates by name in the 2002 Bipartisan Campaign Reform Act and the Supreme Court upheld it in 2003. But on Monday, a more conservative court, by a 5-4 vote, overturned BCRA at least in part. How big a part is open to interpretation — and to possible clarifying action by the FEC and Congress.

Chief Justice John Roberts’ majority opinion can be read narrowly to permit corporate and union funding only of “genuine issue ads” concerning pending legislative matters that do not “expressly advocate” a candidate’s election or defeat or contain the “functional equivalent” by mentioning an election or candidacy and “take no position on a candidate’s character or fitness.”

On the other hand, dissenting Associate Justice David Souter wrote that as a result of the decision “the ban on contributions by corporations and unions and the limitation on their corrosive spending when they enter the political arena are open to easy circumvention, and the possibilities for regulating corporate and union campaign money are unclear.”

Souter noted, quoting Roberts, that an “issue ad” could be banned as “express advocacy” only if it is “susceptible of no reasonable interpretation other than as an appeal to vote for or against a specific candidate.” Indeed, even Associate Justice Antonin Scalia, who voted with Roberts, said his opinion was “vague.”

So, suppose a group of pharmaceutical and insurance companies spends millions on ads in the fall of 2008 declaring “Hillary Clinton Favors Socialized Medicine. Send Her a Message!” Or, the AFL-CIO puts on ads declaring “Free-Trader Fred Thompson Will Send Jobs Overseas. Call Him and Tell Him ‘No.’” Will they pass muster?

The FEC should write rules in advance clarifying the court’s decision, not wait until after the election to pass judgment on ads. Or, Congress could try to clarify what’s allowed and what’s not. Unfortunately, Congress is divided and the FEC is weak.

We fear we’re in for “anything goes.”