There are still 12 months to go, but these margins are very rare in generic-ballot polling.

Buried inside the new CNN poll is a finding that won't make many headlines, but should probably cause a good bit of concern for Republicans.

The poll asked the standard “generic ballot” question: Would you prefer a generic Democrat or a generic Republican in the upcoming election? Democrats lead on that question for the 2018 midterms by a whopping 16-point margin, 54 percent to 38 percent.

If that were actually to turn out to be the case, of course, we'd be talking about a Democratic landslide — and almost definitely a Democratic takeover of the House that is so difficult given the map. But even if it's just close to reality, it could be a very bad omen for the Republican Party in a historically tough first midterm election under a president of their own party.

It is the biggest lead for Democrats on the generic ballot so far this year, but other polls have put them up in the high single digits or even double digits. Below, I've combined all the high-quality, nonpartisan polls that have asked this question since April (with a hand from RealClearPolitics). Democrats have led on every single one, often by a significant margin:

You actually have to go back to 2008 to find a time when Democrats led so big and consistently on the generic ballot. That, of course, was the last big wave election for the Democratic Party — a year in which they took over the presidency and the Senate and built upon the House majority they had won two years before.

Importantly in the new poll, Democrats lead among independents — albeit by a small margin, 44-42. Given more GOP voters come from the ranks of independents and more Democrats generally vote than Republicans, the GOP needs to win independents just to draw even in a given election. And the GOP has won independents by between 6 and 19 points in every election since 2010, according to national House exit polls.

The new poll might paint a bit too rosy of a picture for Democrats, though. For instance, it shows just 1 percent of Democrats crossing over to vote for Republicans, when that number is usually in the mid-single digits. The poll is also weighted a little heavier toward Democrats than most, with 31 percent of respondents coming from the blue side and 23 percent coming from the red side.

It's also true that Democrats generally lead on the generic ballot even when the election turns out to be even. As Will Jordan noted, CNN's polling has had Democrats up in the generic ballot in every election since 2006 -- even though Republicans won big in 2010 and 2014.