The US Supreme Court has eased campaign finance restrictions in a ruling that will allow issues-led groups to run TV adverts right up until elections.
The ruling loosens restrictions on ads in the final days before voting
The 5-4 ruling upheld an appeals court decision that an anti-abortion group should have been allowed to air ads in the 60 days before the 2004 election.
The 2002 bill which imposed the curbs aimed to prevent "attack ads", paid for by lobby groups, targeting candidates.
Chief Justice John Roberts said the law had violated the right to free speech.
The Supreme Court's ruling considerably loosens an important provision of the 2002 campaign finance act, sponsored by Republican Senator John McCain and Democrat Russ Feingold.
Under the bipartisan act, interest groups were banned from airing adverts that named a candidate for office in the 60 days before a general election and 30 days before a primary election.
The Supreme Court has now ruled that such adverts can name candidates, provided that it is in the context of a wider explanation of the issue and without explicitly calling for the candidate to be chosen or rejected.
Not allowing such advertisements infringes First Amendment rights to free speech under the US Constitution, the court said.
Giving the majority decision, Chief Justice Roberts wrote: "Discussion of an issue cannot be suppressed simply because the issues may also be pertinent in an election.
"Where the First Amendment is implicated, the tie goes to the speaker, not the censor."
The court's decision upheld an appeals court's ruling that the Wisconsin Right to Life group should have been able to run certain television ads before the 2004 presidential election.
The interest groups affected by the ruling include corporate and trade union organisations, as well as lobby groups on issues such as abortion, gun ownership and the environment.
The implications are likely to become apparent at the end of this year in the run-up to the 2008 presidential primaries and caucuses in states including New Hampshire, Iowa and Florida.
The McCain-Feingold act was upheld by the Supreme Court, with some different justices on the bench, in a previous challenge in 2003.
Justice David Souter, who was among the four justices to dissent from Monday's ruling, said the 2003 decision had been "effectively and unjustifiably overruled today".
Senator McCain said it was "regrettable" that the court had overturned the provision governing adverts financed by interest groups.
But, he pointed out, the central reform imposed by his 2002 law - preventing political candidates from raising unlimited amounts of "soft money" from unions, companies and wealthy donors - remained in place.