Democrats didn’t make those gains by sweeping the 29 Republican-held districts where John Kerry did better in 2004 than he did nationwide. In fact, Democrats won only 12 of those districts, and just four of the eight districts that fit the aforementioned criteria for where Democrats ought to be favorites.

Instead, they won a significant number of Republican-leaning districts. They nearly won districts in Wyoming and Idaho. Democrats also had some good luck — scandals brought down several Republicans in extremely conservative districts — but many Republicans lost on favorable terrain without any excuse.

Democrats Beating Expectations

Back to 2018. There’s a stronger relationship between a district’s presidential and House vote today than there was a decade ago, so Democrats will probably do a bit better in relatively blue districts and worse in deeply conservative districts. But the same basic idea applies: Democrats will make most of their gains in the big pool of districts where Republicans are expected to have an edge.

No matter how you cut it, none of this year’s special elections clearly fall in the first category of easy targets for the Democrats, even in a wave election. All of these districts voted for President Trump, and each voted for Mitt Romney by double digits in 2012. The Democrats had the advantage of not having to face an incumbent, but that wouldn’t be enough to give them the edge.

Every district and contest is different in some way, of course, making it hard for analysts or statistical models to nail down exact expectations. We could argue all day about whether, on paper, Democrats should be considered one-point or 10-point underdogs in an open race in Georgia’s Sixth in a wave environment. The debate turns on how much weight to give Mr. Trump’s weak 2016 performance — he won by just 1.5 points in that district — compared with the Republican Party’s dominant record in the district in recent statewide elections, House contests and prior presidential races.

But even a favorable analysis for the Democrats would probably make the district a true tossup. Let’s assume, for the moment, that Mr. Trump’s performance is by far the most important measure of the district’s partisanship. Even by this aggressive measure, Mr. Ossoff would still be a one-point underdog in a wave election. Indeed, similar races have split almost 50-50 in wave election years, like 2006 or 2010.