The Supreme Court on Monday refused an emergency appeal by Pennsylvania Republicans to block the implementation of a new court-ordered congressional map, a decision that all but assures that new district lines that make several races more competitive for Democrats will be in place for this year's midterm elections.

The new boundaries are a boon for Democrats, who now have a stronger shot in as many as seven Pennsylvania congressional seats as they look to net 24 seats nationwide to win back the House.

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Pennsylvania Republican Reps. Keith Rothfus Keith James RothfusCNN's Tapper tried to talk GOP candidate out of running against Democratic incumbent: report Lobbying world Conor Lamb gets 2020 challenger touted by Trump MORE, Ryan Costello Ryan Anthony CostellloTrump struggles to stay on script, frustrating GOP again Bottom line Former GOP Rep. Costello launches lobbying shop MORE and Brian Fitzpatrick Brian K. FitzpatrickFlorida Democrat introduces bill to recognize Puerto Rico statehood referendum DCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program 2020 Global Tiger Day comes with good news, but Congress still has work to do MORE all face far tougher roads to reelection under the new map. And the seat currently held by Rep. Patrick Meehan, who is retiring amid allegations of sexual harassment, is now seen as a likely Democratic pickup.

The boost for Democrats in those four districts — as well as improved numbers in other districts that are still seen as reaches for the party — comes as Democrats are increasingly emboldened that they can take back the House majority in 2018.

“Pennsylvania Republicans now in a world of trouble. Dems already flipped one seat and, with the new map, will flip more,” Rep. Ted Lieu (D-Calif.) tweeted Monday, summarizing the Democratic Party’s optimism about the new map.

The state's congressional district map has been at the center of controversy for months. The issue took center stage when the state Supreme Court ruled the old map an unconstitutional gerrymander and ordered it redrawn earlier this year.

The state court gave the GOP-controlled legislature and the Democratic governor three weeks to agree on new lines. When the two sides failed to reach a compromise, the state Supreme Court drew its own map.

Republicans have been challenging the map in court, arguing that the state Supreme Court erred by not giving the state legislature enough time to agree on a replacement map. But the GOP was rebuffed twice on Monday when it sought relief from two federal courts.

First, a three-judge panel with the U.S. District Court for the Middle District of Pennsylvania ruled against a case brought by top Republican officials, in the state as well as eight Republican congressmen.

“The Plaintiffs' frustration with the process by which the Pennsylvania Supreme Court implemented its own redistricting map is plain," the judges wrote in their opinion.

"But frustration, even frustration emanating from arduous time constraints placed on the legislative process, does not accord the Plaintiffs a right to relief."

Hours later, the U.S. Supreme Court announced that Justice Samuel Alito had presented the GOP's emergency request to the court, which denied it.

That effectively halted all of the GOP’s means of recourse. The Supreme Court's denial of the emergency stay can't be appealed, and it didn’t release any dissenting opinion that could suggest a pathway forward.

While plaintiffs could theoretically challenge the district court's ruling, the final say would rest with the Supreme Court, which has already refused the emergency motion.

Time has also nearly run out for Republicans hoping to avoid a boundary change. Tuesday marks the filing deadline for congressional races, a fact the Supreme Court almost certainly took into account when it denied the appeal.

Taken together, legal experts say the justices sent a clear signal that the newly drawn map will stand.

Republicans seem to agree.

“We continue to believe that the Pennsylvania Supreme Court overstepped its authority in an unprecedented fashion when it legislated from the bench,” state House Speaker Mike Turzai said in a statement to HuffPost.

“Nonetheless, we respect the decision of the U.S. Supreme Court and are prepared to move on to other issues of importance to the people of Pennsylvania.”

The decision will have a significant impact on the November elections, let alone control of the House. It could also prompt Republican lawmakers to change their minds and skip a reelection bid, although the state’s filing deadline is the end of the business day on Tuesday.

Meehan’s seat had already been privately chalked up as a tough climb at best for Republicans or a non-starter at worst even before the map changed. Democratic presidential candidate Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE won the district in 2016 by 2 points, and having the sitting congressman retire in the face of a sexual harassment allegation was only going to improve Democratic chances.

But now, the new lines for the seat Meehan is vacating are even more Democratic, to the point that the non-partisan Cook Political Report considers the seat “likely Democratic.”

The race also spells significant trouble for Costello, Rothfus and Fitzpatrick, who are running in districts that Cook rates as in toss-up seats.

Costello had already been expecting a tough race in a seat Clinton won by less than a percentage point in 2016. But analysis by The New York Times found that Clinton would have won that new seat by 10 points.

Fitzpatrick had also been gearing up for a tight race under the old lines. But the new lines shift his district further in favor of Democrats.

Rothfus had previously been expected to cruise to reelection in a solid Republican seat that President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE won by more than 20 points. But the new lines put him in a seat that the Times estimates would have only backed Trump by 3 points.

He’ll likely face Conor Lamb, the Democratic candidate fresh off an apparent upset victory in last week’s special election in suburban Pittsburgh.