If the challenge is successful, it could introduce hundreds of millions of new dollars into the federal election system. The court's decision in 2010's Citizens United vs. FEC, along with a lower court ruling, ended restrictions on political contributions from the general funds of corporations, unions and individuals for "independent" electioneering.
The new case was brought by Shaun McCutcheon of Alabama and the Republican National Committee to challenge "aggregate limits" -- the restrictions on the total amount of contributions that can be given directly during a particular two-year election cycle.
The Federal Election Campaign Act sets up separate limits on the amounts individuals may contribute to federal candidates and other political committees, some indexed for inflation.
But the FECA also sets an overall limit on the aggregate amount individuals may contribute in a two-year period, both to non-candidate linked committees and committees linked to a candidate. Those limits restrict what an individual may contribute to no more than $46,200 to all federal candidates, and no more than $70,800 to federal political action committees and political party committees.
The RNC and McCutcheon seek to break through those barriers on direct contributions.
The New York Times said a majority of the justices -- the five conservatives -- appeared ready to lift the aggregate restrictions. The Los Angeles Times said the outcome likely would depend on conservative Chief Justice John Roberts, who could have the deciding vote.
USA Today said the justices "seemed divided right down the middle," with Roberts searching for a middle ground.
SCOTUSBLOG.com said no consensus was evident.
At a White House news conference, President Obama said the latest challenge to political donation restrictions "says anything goes."
Obama said a lot of Washington's current problems stem from Citizens United, which unleashed "extremist" billionaire donors.
"A big chunk of the Republican Party right now are in gerrymandered districts ... and those folks are much more worried about a Tea Party challenge than they are about the Democrats," pushing the party to the right, Obama said.