Two states where Democrats are looking for a wave to crash in November

(CNN)With Pennsylvania Republican Rep. Ryan Costello confirming his retirement over the weekend, there are now six Pennsylvania Republican House members who have resigned, retired or who are running for another office.

That's more members ditching their seats in Pennsylvania than in all of the elections between 2006 and 2016 combined, according to a CNN analysis of past retirements and resignations. Put another way: more members are retiring in Pennsylvania this year than in the last 10.
Costello's announcement comes on the heels of several tough developments for Republicans in Pennsylvania, including a decision by the Pennsylvania Supreme Court last week to reject the state GOP's plea to reverse a redrawing of the state's congressional districts because they were gerrymandered. Costello had called for the judges who approved the map to be impeached, saying they were "corrupt."
The Pennsylvania Supreme Court's decision, which came only about a week ago, impacted Costello directly. According to an analysis by CNN's Harry Enten, as a result of the the redistricting, two districts -- PA-5 and PA-6 (Costello's district) -- would probably be favored for a Democrat to win. CNN's Key Race ratings place both seats currently held by Republicans as "Likely Democratic."
    There was also Conor Lamb's narrow special election victory in a district President Donald Trump won by 20 points in 2016, which points to how much the political climate has changed from 2016.
    In fact, even before the gerrymandering ruling, a number of Republican members in tightly contested districts had already called it quits ahead of the midterms.
    In September, Rep. Charlie Dent, in PA-16, announced his retirement, later citing Trump as part of his reason for leaving in an interview. "He's a very polarizing figure, and so I suspect our challenges will be even greater just because of that," he told CNN. Dent won his 2016 by double digits but Trump was much less popular in his district.
    Rep. Pat Meehan, whose seat is a possible Democrat pick-up, announced in January that he would not seek re-election. After it was revealed he used taxpayer funds to settle a sexual misconduct claim against him, the Pennsylvania GOP asked him not to return to the House, lest he hurt the party's chances in his district or other competitive districts there. Meehan won his 2016 election by double digits but Trump was not very popular there.
    A similar scenario is playing out in Arizona. At least one Senate seat is opening, leading to several vacant seats -- and the changes have not been free of complications related to Trump.
    Frequent Trump critic Sen. Jeff Flake announced his retirement. Two House members are leaving to run for the Senate.
    Flake has cited his tumultuous relationship with the President as part of the reason for his retirement and President Trump publicly targeted Flake. "It's difficult to move forward in a Republican primary if you have been critical of any of the behavior that's gone on," Flake told CNN.
    Like Pennsylvania, Arizona has also had one leaving member embroiled in scandal related to sexual harassment. Rep. Trent Franks announced his resignation after the House Ethics Committee opened an investigation into allegations of sexual harassment against him. A special election to replace Franks' seat will occur in April.
    Losing members is not just logistically complicated (it can be sometimes be difficult to find good replacement candidates). It's bad politics. There is typically an "incumbency advantage" in elections of a few additional points.
    Those few points could be the difference between who is in power in the House come the next congress.
    If there is to be a Democratic wave, it could be building in Pennsylvania and Arizona.