Pennsylvania is the most compelling of the states: Redistricting and retirements have made it one of the key states in determining which party controls the House after November's midterm elections.

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For the second straight week, voters in four states will choose major party nominees for races with big stakes in November's midterm elections. Pennsylvania, Nebraska, Idaho and Oregon hold primary elections Tuesday for races for the House, Senate and governor. The busiest slate will take place in swing state Pennsylvania, where a new congressional map has made Democrats more competitive in several House districts. Democrats and Republicans will pick candidates for a handful of House races crucial to November's battle for the chamber. If Pennsylvania swings the Democrats' way, the party can pick up a good chunk of the 23 seats it needs to win to take control of the House in November. Here are the races to watch on Tuesday:

Pennsylvania is a House battleground

Earlier this year, the Pennsylvania Supreme Court struck down a congressional map drawn by Republicans. Over GOP objections, the court redrew districts. The changes made the state more hospitable for Democrats. Nonpartisan election analysis site Cook Political Report rates seven GOP-held seats as competitive or favoring Democrats. Another nonpartisan site, Sabato's Crystal Ball, lists six Republican-held districts in those categories.

Republican departures have helped the minority party. In three important races in the state, a GOP incumbent has either resigned or announced his intention to retire. Several House districts in Pennsylvania will feature competitive races either on Tuesday, in November or both. PA 1st District: Democrats are vying to face GOP Rep. Brian Fitzpatrick in a district that shifted in favor of Democrats under the revised map. In 2016, Hillary Clinton narrowly won the new district. Multimillionaire philanthropist Scott Wallace, a former counsel for Senate committees, has piled $2.5 million of his own money into the Democratic primary. The other leading contender is former Navy prosecutor Rachel Reddick. She is endorsed by Emily's List, the organization that aims to elect pro-choice Democrats. On the Republican side, Fitzpatrick faces one challenger in lawyer Dean Malik. The congressman is expected to win. Nonpartisan election handicappers rate the November race as a toss-up.

Democrats are vying to face GOP Rep. Brian Fitzpatrick in a district that shifted in favor of Democrats under the revised map. In 2016, Hillary Clinton narrowly won the new district. Multimillionaire philanthropist Scott Wallace, a former counsel for Senate committees, has piled $2.5 million of his own money into the Democratic primary. The other leading contender is former Navy prosecutor Rachel Reddick. She is endorsed by Emily's List, the organization that aims to elect pro-choice Democrats. On the Republican side, Fitzpatrick faces one challenger in lawyer Dean Malik. The congressman is expected to win. Nonpartisan election handicappers rate the November race as a toss-up. PA 5th District: The state's redistricting made the 5th District dramatically more Democratic. Cook Political Report's Partisan Voter Index, or PVI, which gauges how areas vote in recent presidential elections relative to the country as a whole, now lists it as "D+13." The seat is currently vacant, as Republican Rep. Pat Meehan resigned after the revelation that he used taxpayer money to settle a sexual harassment claim made by a former staffer. Ten Democrats will jostle for the nomination Tuesday. Whoever wins the primary will become the heavy favorite to win the House seat in November. Top contenders include attorney Mary Gay Scanlon, former Philadelphia Deputy Mayor Richard Lazer and former federal prosecutor Ashley Lunkenheimer. Pearl Kim, a former prosecutor in the Pennsylvania Attorney General's office, is the lone Republican candidate.

The state's redistricting made the 5th District dramatically more Democratic. Cook Political Report's Partisan Voter Index, or PVI, which gauges how areas vote in recent presidential elections relative to the country as a whole, now lists it as "D+13." The seat is currently vacant, as Republican Rep. Pat Meehan resigned after the revelation that he used taxpayer money to settle a sexual harassment claim made by a former staffer. Ten Democrats will jostle for the nomination Tuesday. Whoever wins the primary will become the heavy favorite to win the House seat in November. Top contenders include attorney Mary Gay Scanlon, former Philadelphia Deputy Mayor Richard Lazer and former federal prosecutor Ashley Lunkenheimer. Pearl Kim, a former prosecutor in the Pennsylvania Attorney General's office, is the lone Republican candidate. PA 7th District: Democrats got two breaks in the 7th District congressional race. First, redistricting made the area more favorable. By Cook's PVI, it is now a "D+1" district. Then, incumbent GOP Rep. Charlie Dent resigned. While the seat became more competitive for the party, discord could complicate the Democrats' chances. Northampton County District Attorney John Morganelli, a leading Democratic contender, opposes abortion rights and has praised President Donald Trump in the past. He faces pastor Greg Edwards, a candidate running from the left endorsed by Sen. Bernie Sanders, I-Vt. Former Allentown Solicitor Susan Wild, backed by Emily's List, is also vying for the nomination. Former Lehigh County Commissioner Dean Browning and former Olympic cyclist Marty Nothstein are running on the Republican side. Others to watch: The primary elections in Pennsylvania's 6th and 17th Districts may not carry much drama. But the races for those seats in November will be critical. GOP Rep. Ryan Costello declined to run for re-election in the 6th District, now a "D+2" seat, according to Cook. Chrissy Houlahan, a businesswoman who served in the Air Force Reserve, runs unopposed on the Democratic side. Attorney Greg McCauley faces no opponents in the Republican primary. Houlahan would enter the general election as a favorite. The 17th District features a rarity in congressional elections: two incumbents. Democratic Rep. Conor Lamb won a March special election for the state's previous 18th District but lives in the revised 17th District. Both he and GOP Rep. Keith Rothfus will run unopposed in Tuesday's primaries. Cook rates it as an "R+3" seat — less red than the area Lamb won in March. Read more: Long-distance donors flood Tuesday's Pennsylvania primaries with cash

A tough House race in Nebraska

Two Democrats vie Tuesday for the nomination in Nebraska's 2nd District. The winner will face unopposed GOP Rep. Don Bacon in a race nonpartisan handicappers consider a toss-up. Moderate former Rep. Brad Ashford, who lost to Bacon by 1 percentage point in 2016, aims to win back his seat. He faces nonprofit executive Kara Eastman, who is running to his left.

Trump tries to boost an early ally in PA Senate race

GOP Rep. Lou Barletta, an early Trump supporter who endorsed the president in March 2016, aims to win Pennsylvania's Republican Senate primary on Tuesday. The congressman, who has pushed to crack down on immigration since serving as mayor of Hazleton more than a decade ago, has tried to channel Trump to capture the president's success in the swing state. Trump aimed to give Barletta a late boost, recording a robocall that went out to Pennsylvania voters on Saturday and Monday. Barletta will face state Rep. Jim Christiana in Tuesday's primary. The Republican winner will face Democratic Sen. Bob Casey, who runs unopposed Tuesday. Casey is one of 10 Senate Democrats running in states Trump won in 2016. Still, he has clobbered Barletta in fundraising. Pennsylvania is also more purple than red: President Barack Obama won the state in both 2008 and 2012. Read more: Trump aims to help an early ally take down a well-funded Senate Democrat in swing state Pennsylvania

House seats in Oregon and Idaho appear safe