Donald Trump makes his way to board Marine One on Thursday. (Mandel Ngan/AFP/Getty Images) (MANDEL NGAN/AFP/Getty Images)

One of the tricky things about assessing midterm elections is that they are much more complicated than, say, a presidential race. In a presidential contest, there are two major-party candidates vying for victory in 50 states. Or, really, in about 10 states, depending on the year. In a midterm, each of the 435 seats in the House are contested, with up to 870 candidates from the two major parties. (That number is always lower, since some candidates run unopposed.)

As is the case with states in a presidential year, not all of those 435 seats are likely to switch hands. But scores will. What's more, while in a presidential year you can look at polling for the two candidates nationally and get a sense for how states might vote (though this is, uh, not a perfect system), guessing the winner of particular swing House races based solely on national House polling — the generic congressional ballot — is a whole different beast.

Happily there are sites which go through each race on a near-daily basis and assess how likely it is that each seat will end up being held by the same party after voting concludes. One such site is the Cook Political Report, which does regular ratings of House races on a number of different metrics, offering a sense of how the overall national picture might look after Election Day.

Cook's current ratings offer a very specific sense of how things might look in the 2018 election. In short: Bad for the Republican Party.

The most effective way of showing how bad things look, we realized, is to compare this year to the past three cycles. Cook's ratings are available beginning in the last few months of the 2012 cycle, sorting races into three types: Those which one party or the other is likely to hold, those that a party is favored to hold and those seats which are toss-ups.

From darker (safer) to lighter (toss-ups), here's how Cook's ratings evolved over the last two months that year.

Cook Political House ratings by day, 2012. (Philip Bump)

(The slightly lighter colored sections are seats rated to favor one party that were held by the other party.)

Notice that the Republicans had a lot more races in play by Election Day than the Democrats did. The Democrats had 29 seats that were Cook-rated, eight of them toss-ups. The Republicans had 47 seats in play, with 18 toss-ups.

On Election Day, the Democrats picked up eight seats for a 16-seat swing. The Republicans had more seats at risk — and lost more seats.

In 2014, the picture for the GOP was better.

Cook Political House ratings by day, 2014. (Philip Bump/New York)

By Election Day, the Democrats had 12 more seats that were Cook-rated, and 11 more seats that were rated as toss-ups. The Republicans picked up 13 seats for a 26-point swing.

The 2016 election was once again expected to be bad for the Republicans.

Cook Political House ratings by day, 2016. (Philip Bump/New York)

But it ended up being better than it looked. Republicans had 30 more Cook-rated seats than the Democrats, including 14 more toss-ups. They only lost six seats, though, in an election cycle that revealed some significant gaps in localized polling.

That is probably the good news Republicans might take away from the three graphs above: A bleak picture from Cook doesn't necessarily mean a total blowout. That's good to keep in mind when looking at how things stand for 2018.

Cook Political House ratings by day, 2018. (Philip Bump/New York)

The Republicans, as of Thursday's Cook update, have 57 more Cook-rated seats, including 26 more seats which are considered toss-ups. If both parties lose all of their toss-up races (which isn't usually how things work), the Democrats would see a 52-seat swing in their favor — enough to retake the House.

In total, Cook sees more than a third of the seats held by Republicans as being at risk in November. Most of those seats will be retained by the party. But that's not much consolation for a party looking at potentially losing power in the lower chamber.

The best hopes at the moment? A reversal of the trend favoring the Democrats on the generic ballot that hints at some of these apparently contested Republican seats. Or another surprise election that gives the Republicans more success than anyone expected.

Both are possible. Neither, though, is the most likely thing to expect.