It’s a small difference, but it’s striking in historical context. Republicans have almost always done better among likely voters than registered voters, and in recent midterm elections the gap has been particularly significant — perhaps as much as five or six points.

Democrats might have some additional upside, too. They lead by four points in the tossups among registered voters who say they’re “almost certain” to vote.

Many pollsters use a simple likely-voter model limited to this group. In our turnout modeling, we give everyone at least some chance of voting because, in the past, many people who said they wouldn’t vote ultimately showed up. And there are some people who say they will vote who stay home. Either way, it’s another indicator.

Democrats have the wind at their back

The overall national political environment, which seems to favor the Democrats, does appear to be filtering down to the battleground districts. President Trump has a 42 percent approval rating on average in our seven tossup districts , about seven points beneath the 49 percent he won across the districts in 2016. You could roughly extrapolate that to a 39 percent approval rating nationwide, which is pretty close to recent national surveys.

In the generic ballot — which asks people whether they intend to vote for a Democrat or a Republican for Congress — Democrats have a three-point lead in these districts. It’s a modest advantage, but it’s also impressive for seven districts that voted for Mr. Trump by five points.

Republican incumbents don’t look strong

Republican incumbents haven’t fallen behind in most of the races we’ve polled, but their position doesn’t look too impressive, either.

In the tossup districts, only two Republicans — Andy Barr in Kentucky, who led by one point with 47 percent in Kentucky’s Sixth, and Dave Brat, who led by three points with 47 percent in Virginia’s Seventh — have held more than 45 percent of the vote.