Justices were reviewing a fight from Pennsylvania over a congressional map that essentially redistricted three Democratic House members out of a job. Court members debated whether to throw out the entire map, or order a redrawing of one questionable district in the Philadelphia suburbs.
Democratic voters who challenged the plan want the high court to use the case to spell out rules limiting partisan gerrymandering, the practice of drawing districts to favor one party over another.
During the arguments, Justice Antonin Scalia asked how the court could define unfairness.
Paul M. Smith, the attorney for the Democrats, said that a minority party getting two-thirds of new seats after redistricting is unfair.
But Bart DeLone, a deputy attorney general from Pennsylvania, said that there is political motivation in line-drawing. "And frankly, we don't have a problem with that," he said. "The question is whether or not they have been shut out of the process."
The Supreme Court has made it almost impossible to win a claim that partisan gerrymandering is unconstitutional, although justices left the door open to such claims in a splintered 1986 ruling.
The case is important because of the high stakes in boundary-drawing for political parties. States must redraw boundaries after every census to reflect population shifts, and redistricting has gotten more sophisticated.
Richard Hasen, an election law expert at Loyola Law School, said justices may be "seriously considering giving partisan gerrymandering claims some teeth." Or, after 17 years, they may just feel it's time to revisit the subject, he said.
If the high court backs away from its 1986 ruling, as the GOP is urging it to do, Republicans could lock in their control of Congress for years to come, said Nathaniel Persily, a professor and redistricting expert at the University of Pennsylvania Law School.
But if the court endorses the greater judicial involvement sought by the Democrats, the ruling would probably lead to legal challenges to the new congressional maps in at least a half-dozen other big states, he said.
Smith told justices in a filing that "as line-drawers have become more brazen in their willingness to instill bias in electoral maps, the need for judicial intervention has become compellingly clear." He called the court's attention to a bitter court fight over redistricting in Texas that also involves Democratic charges of blatant GOP gerrymandering.
John P. Krill is the lawyer who represents the Pennsylvania Senate president pro tem and the House speaker, both Republicans. He told justices that redistricting is political and said it would be impossible for judges "tasked with exorcising partisanship from mapmaking" to know how to do it.
Because of Pennsylvania's slower-than-average population growth, the state lost two of its 21 seats in Congress after the 2000 census.
The new 19-seat maps were approved with substantial Democratic support, but they were engineered by the Republicans who control both houses of the Legislature at a time when powerful computer software makes it possible to trace voting patterns down to individual streets and houses.
Largely because of where the district boundaries were drawn, the GOP tightened their grip on the state's congressional delegation, winning 12 of the 19 seats in 2002. Previously, the Republicans had 11 seats to the Democrats' 10.