Retiring Rep. Ryan Costello (R-Pa.). (Zach Gibson/AP)

On Tuesday, Republicans got a reality check on just how much trouble retirements within their ranks can cause them as they try to retain control of the House of Representatives in November.

It happened in the outer Philadelphia suburbs. Rep. Ryan Costello (R) is retiring after two terms in Congress, and he did it in a way that almost certainly hands Democrats the seat.

Costello officially announced his retirement Sunday, blaming President Trump for the decision. A few days earlier, Trump had threatened to veto a spending bill that Republicans heaved over the finish line to keep the government open through September.

“Whether it's Stormy Daniels,” Costello told the Daily Local News on Sunday, “or passing an omnibus spending bill that the president threatens to veto after promising to sign, it’s very difficult to move forward in a constructive way today.”

Costello would also be running for reelection in a district that has been redrawn by the courts to be much more favorable to Democrats. Layer that with his candid take on Trump, and Costello's retirement pretty clearly indicates that he doesn't think he could win in a competitive district right now, not with an environment unfriendly to Republicans and an unpredictable and unpopular president as the face of his party.

Whatever his reasons, Costello had decided to retire after the filing deadline for new candidates to enter the race had passed. That means Pennsylvania Republicans didn't have a chance to put a new candidate on the ballot, at least not without jumping through hoops that involved Costello staying on the ballot through the May primary. If Costello technically stepped aside after being nominated, the state Republican Party could pick someone to take his place. And then it could try to keep this seat, which has been in Republican hands since 2012.

But Tuesday, Costello said he wouldn't help them do that. As the Philadelphia Inquirer's Jonathan Tamari reports, Costello said he will pull his name off that primary ballot — he is retiring, after all  — rather than run in the primary and drop out after he wins. That leaves little-known lawyer Greg McCauley as the only Republican on the ballot. Election analysts at Cook Political Report say he is too conservative for the district, and they give the upper hand to the Democrat in the race, Chrissy Houlahan.

That is a lot of drama for one race, and you arguably don't need to know every little detail. But looking at the big picture, here's what this means: Before the November midterms, Democrats have two near-certain pickups in Pennsylvania. (The first being Democrat Conor Lamb, who won a special election this month in a district that Trump won by 20 points; Lamb is likely to win in the district in November because of the redrawn lines. And, yes, Democrats have a GOP retirement to thank for that bit of good news: That special election was called only because a longtime GOP congressman had resigned his seat over a sex scandal.)

Put another way: We're entering an election cycle in which every pickup for Democrats matters, and Republican retirements are set to be a major factor in their ability to knock off Republicans.

Democrats are hoping to regain control of the House for the first time in eight years, and they need to net 24 seats to do it. (Well, 23 now that Lamb has won.) At the top of their target list are 23 districts that Republicans currently hold that Hillary Clinton won in November. Costello's was one of those, and now they probably don't have to spend nearly as much money or attention trying to kick him out of Congress. Same with competitive races in California, New Jersey and Florida, thanks to retiring GOP lawmakers who have simply had it with Congress and national politics.

As I wrote on Monday:

Since Brookings Institution started tracking retirements in the 1930s, Republicans have never had this many retirements in one election cycle. Republicans in Congress are retiring in historic numbers this year. Nearly 50 Republicans in the House and Senate have announced they are retiring, and 25 of those are retiring without having another job to go to or have already left office, compared with 10 House Democrats.

Retirements are a big reason Democrats are positioned to try to take back control of the House of Representatives for the first time in eight years. In our rankings of the top 10 House races most likely to flip parties, five are Republican seats made more competitive by lawmakers' decision to retire.

Costello's retirement may be the most dramatic example of how Republican retirements are benefiting Democrats this election cycle. But it's far from the only retirement-related break that Democrats are getting.