The Supreme Court said Monday that a lower court should take a new look at a challenge to federal restrictions on political advertisements, delaying a major ruling on the constitutionality of ad limits until after this year's elections.

Justices could have used the case, brought by an anti-abortion group, to spell out when so-called grass-roots ads are allowed at election time.

Without dealing with that issue, the court overturned a decision that barred Wisconsin Right to Life from broadcasting ads that mentioned a senator during his 2004 re-election campaign.

In an unsigned opinion, justices said that the Supreme Court's 2003 ruling upholding a federal campaign finance law left the door open for future challenges that the law, in practice, violated free-speech rights.

"This could be an important first step toward undermining (the 2003 ruling) without overruling it," said Richard Hasen, an election law expert at Loyola Law School.

The case now returns to a three-judge federal panel in Washington, although it could be back before the Supreme Court later this year.

"It is certainly our hope this can be dealt with promptly so that now, not only we will know, everyone will know what type of lobbying ads are permitted," said James Bopp Jr., the attorney for the Wisconsin group.

Wisconsin Right to Life had challenged the part of the 2002 campaign finance law that bans the use of corporate or union money for ads that identify federal candidates two months before a general election. The group's ads named Sen. Russ Feingold, D-Wis., who was up for re-election, and the state's other senator.

The commercials urged people to call the senators and ask them to oppose Senate filibustering of President Bush's judicial selections. Feingold co-authored the campaign finance law with Sen. John McCain, R-Ariz.

The appeal had given the Supreme Court its first opportunity to review the McCain-Feingold campaign finance law in practical use. Justice Sandra Day O'Connor's retirement could leave the court split 4-4 on the law, because she was the deciding vote in the 2003 ruling.

O'Connor was on the bench Monday possibly for the last time. The court is taking a monthlong break.

New Chief Justice John Roberts announced the decision, which was not signed and was unanimous.

The case is Wisconsin Right to Life v. Federal Election Commission, 04-1581.