HARRISBURG — Fights for U.S. House seats are shaping up as the most closely watched contests in Pennsylvania as the Democratic Party aims to recapture the U.S. House majority.

Republican losses in the state wouldn’t be surprising because the party of the president historically does poorly in midterm elections.

But Democrats have another reason to believe they can flip up to six seats and help the party erase the 23-seat Republican House majority. A state Supreme Court decision in January threw out 6-year-old congressional district boundaries as unconstitutionally drawn to benefit Republicans. Under that map, Republicans won 13 of Pennsylvania’s 18 House seats in three straight elections even as Democrats dominated statewide elections.

The replacement districts approved by the court’s Democratic majority are fueling more competitive contests.

A look at key races:

1st District

Freshman Republican Rep. Brian Fitzpatrick is trying to fend off a stiff challenge by Democrat Scott Wallace in a closely divided district in Philadelphia’s suburbs. Fitzpatrick, a former FBI agent who succeeded his brother in the seat, has a centrist voting record and has tried to put distance between himself and President Donald Trump.

Wallace, a longtime Democratic Party donor, was co-chairman of the Wallace Global Fund, a Washington, D.C.-based organization that supports liberal social movements.

It is shaping up as Pennsylvania’s most expensive U.S. House race. More than $7 million has flowed in from national party organizations and other outside groups, primarily to attack Wallace and boost Fitzpatrick’s image. Wallace is heavily funding his campaign and outspent Fitzpatrick nearly 5-to-1 in the July-September quarter.

Democrats have a slight registration advantage in the Bucks County-based district, and Democrat Hillary Clinton eked out a win there in 2016’s presidential contest.

5th and 6th Districts

Democrat Mary Jo Scanlon and Democrat Chrissy Houlahan are expected to win these open congressional seats in southeastern Pennsylvania that now heavily favor Democrats after years of Republican control.

Scanlon, a longtime public interest lawyer, would succeed Republican Rep. Pat Meehan, who resigned from the 5th District seat in April while under an ethics investigation for using taxpayer money to settle a former aide’s sexual harassment complaint.

Houlahan is a former Air Force engineer and athletic apparel company executive. She would succeed two-term Republican Rep. Ryan Costello, who decided not to seek another term in the 6th, which has become less hospitable to Republicans.

7th District

Democrat Susan Wild and Republican Marty Nothstein are vying for an open congressional seat in eastern Pennsylvania’s Lehigh Valley, and some parts of Monroe County, a district whose new boundaries favor a Democrat after two decades of Republican control.

Seven-term Republican Rep. Charlie Dent decided against seeking re-election in a potentially hostile political environment, and he resigned in May. Wild is a longtime lawyer in Allentown, and Nothstein is a gold medal-winning Olympic cyclist in the 2000 Sydney games and a Lehigh County commissioner.

In the redrawn district, Democrats have a 62,000-voter registration advantage. Meanwhile, Democratic Party-aligned groups have spent more than $1.3 million attacking Nothstein or boosting Wild.

8th District

Three-term Democratic Rep. Matt Cartwright in northeastern Pennsylvania is being challenged by Republican John Chrin, a former investment banker who moved from New Jersey to run.

The district is anchored by Scranton and Wilkes-Barre, and includes a portion of Monroe County. Democrats have a 75,000-voter registration advantage there. Still, about half of the district is new to Cartwright and Trump would have won it by nearly 10 points in 2016.

Those dynamics are giving Republicans hope for an upset victory. The National Republican Congressional Committee has spent more than $550,000 on TV ads attacking Cartwright.

10th District

Three-term Republican Rep. Scott Perry, owner of one of the most conservative voting records in the U.S. House, finds himself in a substantially less conservative district and his toughest re-election campaign. Challenging him is Democrat George Scott, a fellow Army veteran.

More than $1.5 million from outside groups has flowed in to the contest, including at least $630,000 to attack Scott from a super PAC that is aligned with Trump. The district, which includes the cities of York and Harrisburg, has a Republican registration edge of about 23,000, but there are also 79,000 independent or third-party voters.

Scott, a Lutheran pastor, says he believes he can win with support from independent voters and Republican moderates who reject Perry’s conservatism.

16th District

Republican Rep. Mike Kelly is seeking a fifth term in a northwestern Pennsylvania district that slightly improved for Democrats when the state Supreme Court redrew it, and he’s got his toughest re-election challenge to date from Democrat Ron DiNicola, a lawyer and former Marine.

Democrat and Republican registration is practically even, although the odds seem to be in Kelly’s favor: Trump won the district by 20 percentage points and held a rally there with Kelly earlier this month. Still, the Democratic Congressional Campaign Committee has poured more than $600,000 into the race to help DiNicola.

17th District

Newly minted Democratic Rep. Conor Lamb is aiming to knock off three-term Republican Rep. Keith Rothfus in Pittsburgh’s suburbs in the nation’s only House race pitting two incumbents against each other.

Democrats enjoy a 71,000-voter registration edge in the district, and Lamb has comfortably led polls over Rothfus, who has one of Pennsylvania’s most conservative voting records in Congress.

The district isn’t necessarily an easy get for a Democrat: Trump would have won it by almost 3 points in 2016.

Still, Lamb has street cred in swing districts: He won a special election in March to succeed Republican Rep. Tim Murphy, who resigned, in a district that Trump won by about 20 points.