In 2006 and 2010, for example, both years in which a party saw a huge wave that allowed it to take control of the House, Cook added more and more seats to its set of “toss-ups” — races that could go either way — as each year went on. Here are the seat ratings a year prior to the 2006 election and right beforehand, for example. The lighter the bar, the more closely contested it was.
The Republicans went from fewer than five toss-up seats in November 2005 to nearly 40 by the time the election rolled around. The Democrats won a bunch of those seats — and control of the House.
On Thursday, Cook announced new House ratings with a similar effect: A number of Republican-held seats moved into play.
We can compare Cook’s ratings to the site’s Partisan Voting Index, a measure of how the districts voted in the last two presidential elections. Most of the Democratic-leaning seats (left of “even”) are fairly solidly Democratic. (The darker the blue, the safer the seat for the Dems.) But in more balanced districts, there are a lot more Republican seats that are in play. The closer you get to “even,” the lighter those shades of red. There are even six Republican-held seats that right now lean Democratic.
As Cook’s David Wasserman noted on Twitter, the Democrats don’t need to win any of those seats that are leaning Republican or likely Republican to take control of the House. If they just hold their own contested seats and win all of the Republican toss-ups, they win the House — by a one-seat margin.
It may seem strange that the blue from Democratic wins on the chart above would bleed so much into the range of seats that lean Republican. If we look just at the results from 2016, though — meaning we exclude the 2012 race where Barack Obama won districts that Hillary Clinton lost — the split overlaps more neatly with the results.
Giving the Democrats all of the seats that lean their way and the Republican toss-ups yields this:
Just a smattering of Democratic seats in districts President Trump won.
You’ll notice that these latter two charts round 2016 results to the nearest multiple of five points. There are a total of 21 seats in which the 2016 margin was less than 2.5 points for either Clinton or Trump. Of those, nine were won by Clinton and 12 by Trump. Seven of the nine won by Clinton are held by Republicans, and six of the 12 won by Trump are held by Democrats. The Democrats don’t even need to win all of those seats to take control of the House, however narrowly.