The U.S. Supreme Court on Monday rebuffed a bid by Republican legislators in Pennsylvania to reinstate a congressional district map struck down by that state's top court as unlawfully biased in favor of Republicans.

A new state electoral map, devised by the Pennsylvania Supreme Court after it invalidated the Republican-drawn districts in January, is seen as giving Democrats a better shot at gaining seats in the U.S. House of Representatives in the Nov. 6 congressional elections in which President Donald Trump's fellow Republicans are seeking to retain control of Congress.

The case involves a practice called partisan gerrymandering in which electoral maps are drafted in a manner that helps one party tighten its grip on power by undermining the clout of voters that tend to favor the other party. The high court in June failed to determine whether partisan gerrymandering violates the U.S. Constitution after hearing high-profile cases from Wisconsin and Maryland.

The justices on Monday rejected the Republican appeal of the Pennsylvania Supreme Court ruling throwing out the previous Republican-drawn map because it violated the state constitution's requirement that elections be "free and equal" by marginalizing Democratic voters.

The high court previously rejected two Republican requests to block the new district boundaries that the state high court issued to replace the old map, which had been in effect since 2011. Republicans have held 13 of the state's 18 U.S. House seats since 2011 despite Pennsylvania being a closely divided bellwether state.

A group of 18 Democratic voters sued in Pennsylvania last year to challenge the Republican-drawn maps. The Pennsylvania Supreme Court struck down the 2011 map and later adopted the new map in time for use during the party nominating contests in May.

The state's Republican legislative leaders urged the justices to intervene and overturn the ruling by the state court, which they accused of usurping the legislature's authority over redistricting. The Democratic voter challengers told the justices it is long-settled law that the U.S. Supreme Court cannot review a state court's interpretation of state law.

State and federal legislative district boundaries are reconfigured after the U.S. government conducts a census every decade. Partisan gerrymandering has been used for two centuries but has become more extreme with the use of computer programs to maximize its effects in a way that critics have said warps democracy.

The Supreme Court in the past has disallowed gerrymandering intended to discriminate against racial minorities but has not curbed partisan gerrymandering.