US Supreme Court justices have broken into their summer recess for the first time in nearly 30 years to hear arguments about political fundraising.
Kenneth Starr (right) argued the case for Mitch McConnell
In another unusual move, the court scheduled four hours of submissions from lawyers, rather than the 60 minutes allowed in most cases.
At issue are new campaign finance rules which ban national political parties from raising unregulated "soft" money, including donations from corporations or unions, and restrict election-time advertisements from special interest groups.
Supporters of the reforms say they are necessary to ensure that politicians work for all their constituents and to cut the danger of influence being wielded by rich contributors.
Opponents say the law restricts free speech, which is guaranteed by the constitution.
The court is expected to rule by the end of the year, and the decision would then affect how money is raised for the 2004 presidential and other elections.
Intrusion v protection
Lawyer Kenneth Starr, representing Republican Senator Mitch McConnell, said the reforms went too far.
The law "intrudes deeply into the political life of the nation", argued Mr Starr, who is best known as the independent counsel who investigated President Bill Clinton over his affair with a White House intern.
He said the law represented a "federal intrusion" by regulating "state and local political activity at the most grass roots level".
The law, sponsored in the Senate by Arizona Republican John McCain and Wisconsin Democrat Russ Feingold, was approved by Congress and signed by President Bush in 2002 after seven years of debate. It took effect in November.
Mr McCain and Mr Feingold sat in the first row in the spectator section, listening intently to the arguments which focused first on unregulated, and therefore unlimited, contributions to political parties and then the restrictions on broadcast ads by corporations and unions right before elections and a ban on donations by minors.
John McCain (left) and Russ Feingold want their law to stand
Solicitor General Theodore Olson, the government's top courtroom lawyer, defended the law as necessary to protect the integrity of the federal elections by preventing corruption.
"Money had become the number one operative driving force," he told the justices. He said the law sought to prevent an erosion of public confidence in the political system.
Correspondents say the case may lead the court into the most important ruling on money and politics since its landmark 1976 decision on campaign finance.
That case was the last time the court heard four hours of arguments on one issue.
The hearing on the McCain-Feingold law was the first time that the court has broken into its summer recess since July 1974, when it denied President Richard Nixon's attempts to withhold tapes of White House tapes during the Watergate scandal.