But this is not a story about the ocean. It is a story about politics, which is where the metaphor comes in. Wave elections — like those in 2006, which handed the House to the Democrats, or 2010, which flipped it back — are not always obvious a year in advance.

Last week, Cook Political Report released its latest assessment of the state of the race for control of the House. There are, according to that analysis, 62 seats that are leaning toward the Democrats or the Republicans and 21 seats that are considered toss-ups that could got either way. The problem for Republicans is that 45 of those 62 and 17 of those 21 seats are held by their party, meaning that they have much more to lose. After all, if each party holds half of its toss-up seats, the Democrats would win 11 races (nine Republican and 2 Democratic) and the Republicans 10 — a gain of seven seats for the Dems.

We were curious how those margins looked a year before other recent elections. Using the Internet Archive, we looked up the Cook ratings a year out for each election since 2006. The graphs below include likely seats (those that the Democrats or Republicans are almost certainly going to hold), lean seats (those that they will probably hold) and toss-ups.

For now, look at the smaller graphs on the right. The Republicans have 43 more seats rated as likely, lean or toss-up than do the Democrats, meaning that the Democrats have a 43-seat advantage in Cook’s analysis. That’s the biggest spread at this point in a cycle since 2010, when the Republicans had a 56-point spread.

But as the election that year unfolded, that spread widened. By Cook’s last rating, the Republicans had an advantage of over 100 seats, a function of the Democrats having more seats at risk.

The Republicans ended up winning 63 seats from the Democrats in 2010, going from being down 77 seats to up 49 — a shift of 126 seats. That pattern — fewer seats in play a year out, a lot right before the election and a bigger-than-that swing — was not an uncommon one. That’s the point: The wave didn’t look as big 12 months before its arrival.

There’s another important pattern to track. Look at what happened in 2006 and 2010, relative to the year prior.

As the election wore on, a lot more seats moved from “likely” to toss-up for the party that lost seats. This makes sense, given what we know happened: The Democrats in 2006 won control of the House, meaning they won a lot of Republican-held seats. In 2006, the Democrats went from a 13-seat to a 48-seat advantage in Cook’s ratings, with 34 Republican seats moving to toss-up over the final 12 months. In 2010, the Democrats saw 32 seats become toss-ups.

There is one type of wave that telegraphs its arrival in advance. During a tsunami, as is now commonly known, the ocean recedes before rushing back into shore (and beyond). How do we extend our political metaphor to incorporate that? Well, let’s just keep an eye on how many more people announce their decision not to seek reelection, shall we? That’s about as close to the ocean receding as we’ll get — and if it keeps happening, it may be a sign that flooding is imminent.

Philip Bump is a correspondent for The Washington Post based in New York. Before joining The Post in 2014, he led politics coverage for the Atlantic Wire.

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