The Republican Party’s polling for the 2018 elections has been looking grimmer and grimmer — and points increasingly toward a possible Democratic wave next year.

The latest tests of the generic congressional ballot — essentially, a poll question asking whether people would vote for a Republican or Democrat congressional candidate next year — have shown mostly double-digit leads for Democrats.

Indeed, Democrats are currently leading RealClearPolitics’ generic ballot polling average by 13 points — their biggest lead of the year.

Obviously, that looks good for Democrats. But to get a sense of just how good, it’s helpful to compare the Democrats’ current lead in the generic ballot to the final generic ballot polls in previous years, according to RealClearPolitics’ averages.

2002: Republicans +1.7, minor change in Republican-controlled House

2004: Tied generic ballot, minor change in Republican-controlled House

2006: Democrats +11.5, wave flips House to Democrats

2008: Democrats +9, wave further increases Democratic House majority

2010: Republicans +9.4, wave flips House to Republicans

2012: Republicans +0.2, minor change in Republican-controlled House

2014: Republicans +2.4, gains in Republican-controlled House during national GOP wave

2016: Democrats +0.6, minor change in Republican-controlled House

2018: Democrats +13

It is clear that the polling right now most resembles the wave years of 2006, 2008, and 2010. You’ll also notice that Democrats’ current generic ballot lead is bigger than in any of those years.

Related One chart shows Democrats are now more eager than ever to run for office

We’ve also had a year’s worth of elections that Democrats overall did quite well in. According to Daily Kos Elections’ tracking of special elections all over the country (both congressional and state legislature), Democrats have performed on average 10 points better than Hillary Clinton did in their respective districts. So that’s another data point suggesting Democrats are favored to do very well indeed next year.

How many House seats might be vulnerable?

Of course, voters can’t cast a ballot for “Democrats” or “Republicans.” They’ll have to vote for particular candidates in particular districts across the country. But midterm elections have been nationalized in recent years — the last three were all partisan waves — making these, essentially, the baseline background conditions that all those candidates will have to deal with.

Another potential problem for Democrats is gerrymandering, since many key states in the House of Representatives have their district lines drawn to advantage Republicans. And some GOP members of Congress in certain states may well be saved from a wave because of that.

But if these generic ballot numbers hold up — or even moderate a bit — Democrats will have more than enough targets of GOP-held House seats to make a takeover look very plausible.

Republicans currently have a 241-seat majority, and Democrats need to pick up, on net, 24 seats to flip the balance of power. That’s the magic number: 24.

To start off, already there are 23 House Republicans representing districts Hillary Clinton won in 2016. It’s true that these Republicans managed to hold on then, but still, based on partisanship alone, these districts that would be promising pickup opportunities even if there was very little change in the national environment from that year.

The polls and special election results, however, currently suggest a major change in the national environment that could put many seats Trump won in play.

In helping figure out which seats these might be, one metric that could be useful is the Cook Political Report’s Partisan Voter Index. Essentially, this is a measure of the underlying presidential-level partisanship of each district, based on the past two elections — it measures how much more Democratic or Republican each district voted in presidential elections compared to the nation as a whole. (One potential advantage this metric has is that it’s not entirely pegged to the 2016 Trump/Clinton results, and incorporates how districts chose between Obama and Romney in 2012 too.)

So in addition to the 23 Republican-held districts Clinton won, there are:

30 Republican-held seats with a PVI of Republican+5 or below (essentially, the next plausible batch of targets in a pro-Democratic year)

31 Republican-held seats with a PVI of Republican+6 to Republican+8 (seats that could potentially be endangered in a really big wave)

30 Republican-held seats with a PVI of Republican+9 to Republican+10 (seats that would probably only be vulnerable to a tsunami-like wave, or in case of scandal)

Again, the magic number for Democrats is 24. So if there is a big wave in their favor, they’ll certainly have far more possibilities than that among these various groupings of seats.

It is important to keep in mind, again, that candidates matter. Some of these districts are represented by well-liked Republican incumbents who would be tough to dislodge even during a wave. And Democrats will have to find effective challengers.

Yet it’s also true that many of these Republican incumbents have never been tested in an unfavorable national environment, since two-thirds of the party’s House members were first elected to the chamber in 2010 or later. And the willingness of nearly every Republican to vote in support of the party’s very unpopular agenda — both for Obamacare repeal, and for the tax bill — will give them some big new vulnerabilities.

Finally, Democrats will of course have to hold on to their own seats too. It should be noted that there are 12 House Democrats who represent states Trump won. Other seats could become competitive because of scandal. However, in partisan wave elections, parties have generally end up holding on to most of their own seats.

Again, the midterms are still 10 and a half months away, and we can’t know exactly how the political environment will change in that time. (Though according to the Upshot’s Nate Cohn says that in the past, the generic ballot usually hasn’t moved that much after this point). But for now, the data points in Democrats’ favor in 2018.