WASHINGTON — The Supreme Court’s latest ruling on campaign-finance reform, issued yesterday, is only three pages long. But it managed to cast a significant shadow on the Court’s 298-page landmark ruling in McConnell v. FEC.

That 2003 ruling upheld virtually all of the McCain-Feingold campaign-finance law by placing a high premium on the law’s bright-line rules and comprehensive scope. To chip away at the law provision by provision would sap its overarching strength, the Court seemed to suggest.

The Court in McConnell even imposed a less rigorous standard of review than it usually uses in First Amendment cases, in deference to what the majority described as “Congress’ ability to weigh competing constitutional interests in an area in which it enjoys particular expertise.”

But yesterday’s ruling seemed to chart a new direction, opening the door to the very exceptions and line-by-line scrutiny that the Court once discouraged. And the justices did so unanimously.

In a case argued just six days earlier, the justices ruled in Wisconsin Right to Life Inc. v. Federal Election Commission that a lower court and the Bush administration were wrong to foreclose individual as-applied challenges to the provision of the law restricting the broadcast of pre-election issue advertising defined as “electioneering communications.”

Whereas in the 2003 McConnell ruling the Court readily upheld the law on its face, now the justices were insisting the door was open to challenges that attack the law as applied in the real world.

That invitation could get the Court back into the business of carving out exceptions to the law, thereby undermining it, say some election-law experts.

“If the Supreme Court has decided to sacrifice some clarity for greater immunity from campaign-finance regulations, where will be the stopping point?” asked Edward Foley, an Ohio State University law professor. “It is conceivable that, after some years of observing an ‘unworkable’ standard for defining the category of messages eligible for an as-applied exemption, the Court scraps the effort altogether and strikes down the ‘electioneering communication’ funding restriction across-the-board, thereby overruling McConnell.”

Adds Richard Hasen, a professor at Loyola Law School in Los Angeles, “This could be an important first step toward undermining McConnell without overruling it.”

Still, reform advocate Fred Wertheimer, president of Democracy 21, cautioned that “The Court decision did not invalidate the McCain-Feingold law in any respect, and the law remains in place for the 2006 election.”

The Court remanded the case to a three-judge panel in Washington, D.C., with instructions to take another look at the Wisconsin group’s challenge.

The McCain-Feingold law prohibits unions and corporations — including non-profit corporations — from directly funding issue advertising that refers to candidates within 60 days of an election. Those ads can be paid for through political action committees whose funding is regulated and disclosed, but the Wisconsin group says it was entitled to pay for the ads directly while reserving limited PAC funds for other messages.

Wisconsin Right to Life said the law forced it to withdraw what it described as “grass-roots issue advertising” just because Russ Feingold, one of the two Wisconsin Democratic senators mentioned in the ads, was running for re-election. The advertising was aimed at expressing opposition to Democrats’ filibustering of judicial nominees. This kind of advertising, unrelated to an election, should be exempted from the rule set out in McCain-Feingold, lawyers for Wisconsin Right to Life told the Court.

Both the three-judge panel and the Bush administration, in defending the law against the Wisconsin group’s challenge, said the McConnell ruling foreclosed such as-applied challenges.

But during oral argument last week, Chief Justice John Roberts seemed angry at the government for taking that stance, which he said amounted to a “classic bait and switch” because the government in pre-McConnell filings had said as-applied challenges were possible.

Roberts’ concerns were reflected in the unsigned per curiam opinion that he announced from the bench yesterday. “In upholding [the law] against a facial challenge, we did not purport to resolve future as-applied challenges,” the ruling stated.

The decision came on what could be Justice Sandra Day O’Connor’s final day on the bench before retiring. The decision could have been a way for the justices to put off hard questions raised in the case until after O’Connor’s putative successor Samuel Alito Jr. is confirmed — which could come as soon as this week. But in the meantime, the ruling has cast uncertainty over the future course of campaign-finance regulation.