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HARRISBURG, Pa. — Pennsylvania's high court issued a new congressional district map for the state's 2018 elections on its self-imposed deadline Monday, all but ensuring that Democratic prospects will improve in several seats and that Republican lawmakers challenge it in federal court.

The map of Pennsylvania's 18 congressional districts is to be in effect for the May 15 primary and substantially overhauls a congressional map widely viewed as among the nation's most gerrymandered. The map was approved in a 4-3 decision.

Most significantly, the new map likely gives Democrats a better shot at winning seats in Philadelphia's heavily populated and moderate suburbs, where Republicans had held seats in bizarrely contorted districts, including one labeled "Goofy Kicking Donald Duck."

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The redrawn map could boost the Democratic Party's quest to capture control of the U.S. House and dramatically change Pennsylvania's predominantly Republican, all-male delegation.

Related: Supreme Court won’t block new Pennsylvania congressional maps

Meanwhile, sitting congressmen, dozens of would-be candidates and millions of voters have to sort out which district they live in barely a month before the candidates' deadline to submit paperwork to run.

Republican lawmakers are expected to quickly challenge the map in federal court, arguing that legislatures and governors, not courts, have the constitutional responsibility to draw congressional maps.

The Democratic-majority state Supreme Court ruled last month in a party line decision that the district boundaries unconstitutionally put partisan interests above neutral line-drawing criteria, such as keeping districts compact and eliminating municipal and county divisions.

Voters enter the polls to vote in the presidential election on November 8, 2016 in Oakmont, Pennsylvania. Jeff Swensen / Getty Images

The decision is the first time a state court threw out congressional boundaries in a partisan gerrymandering case and handed a victory to the group of registered Democratic voters who sued last June in a lawsuit backed by the League of Women Voters.

Candidates can start circulating petitions to run in their new district in a little over a week, Feb. 27. Pennsylvania has seen a surge in interest in running for Congress with six incumbents elected in 2016 not running again — the most in four decades — and Democrats vehemently opposing President Donald Trump.

Pennsylvania's Republican delegation has provided a crucial pillar of support for Republican control of the U.S. House since 2010.

Republicans who controlled the Legislature and governor's office after the 2010 census crafted it to elect Republicans and succeeded in that aim: Republicans won 13 of 18 seats in three straight elections under the now-invalidated map, even though Pennsylvania's statewide elections are often closely divided and registered Democratic voters outnumber Republicans.

The new map will not apply to the March 13 special congressional election in southwestern Pennsylvania's 18th District between Republican Rick Saccone and Democrat Conor Lamb.