Evan Vucci/AP
CNN —  

President Donald Trump scored a 41% approval rating compared with a 53% disapproval rating, among all Americans in CNN’s latest poll.

That -12 point net approval rating gives us a fairly good idea of what most Americans think of the job Trump is doing. And it’s not good.

But for understanding midterm elections – which are, in a major way, a referendum on the President – it is much more important to look at his approval rating among voters.

CNN’s latest poll looks like most others on this measure. Trump’s approval rating among voters is 44%, while his disapproval rating is 51%. (This shift from -12 points to -7 points is not too surprising given that older people are more likely to be voters than younger people are, and Trump does best with the old and worst with the young.)

While the contrast between how all adults and how registered voters feel may not seem large, a 5-point gap could be the difference between a Democratic House majority and the Republicans staying in power.

That’s because there’s a high correlation between Trump’s approval rating and where Republicans and Democrats stand on the generic congressional ballot.

In the latest CNN poll, Democrats are leading on the generic congressional ballot by only 3 percentage points. In the previous poll, that lead was 6 percentage points.

This tracks pretty close to Trump’s net approval rating among voters.

Democrats need a net gain of 23 seats to win control. Although it is difficult to calculate how much Democrats need to win the House national vote by (which the generic congressional ballot is roughly measuring) in order to win a majority of seats, 7 points seems to be a consensus estimate with a wide margin of error. Republicans, of course, benefit from how the district lines are drawn in this country and the fact that incumbents tend to outperform the fundamentals of their district.

Indeed, a rough guide is that Democrats will pick up an additional three to four seats for every point they gain in the national vote. (This is based off the partisan leanings of each district.) Put another way, they’d probably pick up 20 to 25 seats if they win the House popular vote by 6 or 7 points. They would pick up closer to 35 seats if they win it by 10 points. They pick closer to between 10 and 15 if they win it by only 3 points.

The good news for Democrats is they aren’t likely to lose too much additional ground once pollsters switch over to “likely voters” from all voters like they did in 2010 or 2014. Democrats are enthusiastic about this midterm in a way they weren’t in those years. That matches prior years, which suggests that Democrats should do about equally as well among likely voters in the House vote as they do among registered voters.

The bad news for Democrats is Trump’s popularity among all voters may not be low enough for them to take control of the House.