WASHINGTON — Democrats say Donald Trump is their key to flipping a lot of GOP-held House seats, but their political calculation doesn’t give much weight to races in Pennsylvania, where they are allowing five Republican incumbents to run unopposed.

Redistricting has made it difficult to recruit candidates in some of Pennsylvania’s reddest districts, so the party is concentrating on winnable districts where strong Democrats are willing to run, said U.S. Rep. Ben Ray Lujan of New Mexico, chairman of the House Democratic Campaign Committee.

“We worked in some of the districts that we thought were going to be the most competitive, and that’s where we put our attention,” Mr. Lujan said.

Among them is Pennsylvania’s 8th Congressional District, where control of the seat has switched between the parties six times since 1977. Now Democrat Steve Santarsiero is battling Republican Brian Fitzpatrick, who is trying to keep the seat in the family. It is currently occupied by his brother, U.S. Rep. Mike Fitzpatrick, R-Bucks.

It’s among the seats in the DCCC’s sights.

But the DCCC seems to have given up on five other seats in Pennsylvania, more than in any state except Texas, which has twice as many districts.

That means Tim Murphy of Upper St. Clair, Mike Kelly of Butler, Scott Perry of York, Bill Shuster of Bedford and Tom Marino of Lycoming County have a clear path to re-election in November unless a strong independent or minor party candidate emerges by Pennsylvania’s Aug. 1 filing deadline, and that’s unlikely.

Rob Gleason, chairman of the Pennsylvania Republican Party, said Democrats should have put up candidates.

“The people should have a choice. You have to hold elected officials accountable whether they’re Republican or Democrat and elections are how we do that,” he said. “I think it’s wrong the Democrats don’t have someone running against our party.”

Mr. Gleason’s party, though, is also letting two Democrats run unopposed, giving a clear path to re-election for incumbent Democrats Mike Doyle of Forest Hills and Brendan Boyle of Montgomery County.

That’s different, Mr. Gleason said. Those districts lean so far to the left there’s no point in trying.

The Cook Partisan Voting Index, which rates districts’ political leanings according to voting in past presidential cycles, pegs Mr. Doyle’s district at D+15, meaning the district votes for Democrats 15 percent more than the nation does as a whole. Mr. Boyle’s district is rated D+13.

It’s hard to recruit candidates in those two districts, Mr. Gleason said.

“We couldn’t recruit anybody but we tried,” he said. “We try hard every time.”

So do Democrats, Mr. Lujan said.

Mr. Shuster’s district is R+14 on the voting index. Mr. Marino’s district is R+12, Mr. Murphy’s is R+10, Mr. Perry’s is R+9 and Mr. Kelly’s is R+8.

With a rating of R+1, Mr. Fitzpatrick’s district is considered the most competitive in Pennsylvania, and both parties are expected to invest heavily in the race.

“We do have candidates for the most competitive seats in Pennsylvania. We can always do better,” Mr. Lujan said. Getting candidates to run in every district in the country “is something we can always strive for. When we have the strong list [of candidates] that we have today, that’s something I’m proud of, but we can always build upon it.”

It isn’t easy, particularly since congressional lines were redrawn in 2010 redistricting, said DCCC spokeswoman Meredith Kelly.

“In this very, very gerrymandered world if people don’t believe on the ground that there’s any chance a Democrat can win, we can go to all the right people and if they don’t believe [they can win] they’re not getting into the race,” Ms. Kelly said. “If they don’t believe they can raise the money to step up to an incumbent that has $2.5 million, they’re not getting in the race no matter what.”

When recruitment is successful, sometimes party leaders have to talk to 30 or more people before they find one willing to run, she said.

Nationally, as many as 21 Republicans and 27 Democrats could go unchallenged in House races across the country. In some states it’s too late to declare candidacies, but in others filing deadlines haven’t yet passed, and the DCCC is still recruiting there, Mr. Lujan said.

Mr. Lujan said he expects strong results in districts where they’ve managed to put up candidates. He attributes that to momentum created by Mr. Trump.

When the Republican front-runner has been “insulting to you and your family with every word that he says, that upsets you and then you get out to the polls and help us make sure that we not only prevent Donald Trump from becoming the president, but that we defeat the Republicans in the House that paved a path for the rise of Donald Trump. That’s what this is about,” Mr. Lujan said. “If Donald Trump continues to speak and just be himself he’s going to motivate our base more than anybody else.”

And Democrats across the country intend to capitalize on that, Mr. Lujan said.

Mr. Trump’s effect on down-ballot races has put a lot of seats up for grabs that Democrats never expected to be in contention for, he said.

“Donald Trump is starting to create some movements in the waters that are creating more and more momentum that there would have been without him, and that’s a positive thing,” Mr. Lujan said. “There’s no doubt I see Republicans losing seats in the Congress this year.”

He said Democrats might pick up the 30 seats needed to take back control of the House.

Mr. Gleason said that’s “wishful thinking,” and any seats Democrats might pick up won’t be in Pennsylvania.

“We’re a blue state and we [Republicans] have 13 out of 18 congresspeople, and they’re not new. They’ve been there for a while. … I expect all of our people to back [to Washington]. They run their own campaigns hard-nosed, fighting for every vote,” he said. “If they want to take seats back they’re going to have to look to other states.”

Washington Bureau Chief Tracie Mauriello: tmauriello@post-gazette.com; 703-996-9292 or on Twitter @pgPoliTweets.

First Published: May 1, 2016, 4:00 a.m.