A little over a year out, Democrats' prospects of winning back the House look considerably brighter in nationwide polls than they do in a race-by-race analysis. Since June, the FiveThirtyEight average of congressional generic ballot polls has consistently given Democrats roughly an eight point edge — right about where they would need to be to win House control. But today, it's still very hard to see which vulnerable GOP seats would get them there.

If Democrats were to hold all of the seats we rate as leaning towards them, all 12 of the Toss Ups, and half of the seats in Lean Republican — a big ask — they would still fall two seats shy of a majority.

That's why in addition to an anti-GOP wave, Democrats need a few helpful breaks. The last three times the House switched hands — in 1994, 2006 and 2008 — retirements and scandals in previously out-of-reach districts gave the party out of power critical momentum. GOP Rep. Tim Murphy's ignominious exit this week may give Democrats a glimmer of hope in very red PA-18, but Democrats could especially use retirements in more marginal districts.

Today, there are 52 Republicans sitting in marginal districts with Cook PVI scores of R+5 or more Democratic — more than twice the number Democrats need. The problem for Democrats is that these tend to be personally popular members: three quarters of them won their 2016 races by double digits, and half of them won by more than 15 points. For the most part, they know how to navigate tough districts and attract independent voters.

Of the 52, just four have announced retirements so far: Reps. Ileana Ros-Lehtinen (FL-27), Dave Trott (MI-11), Charlie Dent (PA-15) and Dave Reichert (WA-08). Their retirements have vaulted these seats to the top of Democrats' takeover list. But as the holidays near and retirement season looms, Democrats' chances hinge on how many of the other 48 stick around.

The New York Times's Nate Cohn wrote an excellent piece outlining how difficult incumbents are to beat, even in waves. To build on that analysis, we looked back at the losing party's retention rates in incumbent vs. open seats in the past four "wave" elections: 1994, 2006, 2008 and 2010.

Incumbent Party's Retention Rates in Past Four House Wave Elections

When the District's PVI Favors... Incumbent Running Open Seats Challenging Party by 10 Points or More 33% 0% Challenging Party by 5-10 Points 58% 0% Challenging Party by 0-5 Points 59% 6% Incumbent Party by 0-5 Points 77% 31% Incumbent Party by 5-10 Points 94% 81% Incumbent Party by 10 Points or More 99% 88%

This recent history suggests that even when faced with wave elections, strong incumbents in tough districts still win at impressive rates. But when an incumbent in a district favoring the opposite party retires, that seat is as good as gone in a wave. In the context of 2018, this suggests Republicans' chances of holding onto Ros-Lehtinen's open FL-27 (D+5) will be almost nonexistent if Democrats are doing well nationally.

But, where GOP retirements would make the biggest difference is in the D+5 to R+5 range. For example, when an incumbent runs for reelection against a wave in a district that favors his or her party by zero to five points, he or she wins about 77 percent of the time. But when that incumbent retires, his or her party's retention rate falls to 31 percent. That's why more openings in any of these districts would mean so much to Democrats' 2018 chances.

Of the following 48 districts fitting that description, only 7 are already in the Toss Up column. Most are held by relatively junior Republicans and there aren't obvious open seat possibilities — except Michigan's 6th CD, where GOP Rep. Fred Upton (MI-06) is considering a Senate bid. But if at least five of these members were to step aside in the next several months, Democrats' route to the majority would become much more realistic:

GOP Incumbents in Districts with a PVI Score of R+5 or More Democratic