The Forecast with Harry Enten: Democrats poised to take the House, GOP likely retains control in Senate

We'll be letting you know, based on our data analysis, who is favored to win and lose in House and Senate races through the election. We'll be aggregating those individual estimates to forecast who has the best chance of controlling the House and Senate. These forecasts will be updated at least daily, so check back often!

Right now, the forecast points to a split in control of the next Congress. Democrats need a net gain of 23 seats to win back the House and are forecast to have a net gain of about 30. They need a net gain of two seats to win back the Senate, but, if anything, Republicans are more likely to gain a seat.

It's important to keep in mind that both of these results are within "the margin of error." That is, it's within our forecast that other possible outcomes could occur. Democrats may win both bodies of Congress, or Republicans could win both. We just don't think that is the most likely outcome.

Keep in mind this is the first time anyone at CNN has gotten into forecasting. This is an experimental product, not an official CNN forecast. We don't expect to get every race right. You can read more on our methodology here.

On the House side, Democrats look like they could pick up seats in many different parts of the country.

They're forecast to win a number of seats in the Northeast. Pennsylvania is expected to be a bonanza, with its 5th, 6th, 7th and 17th districts all forecast to flip to the Democrats by 5 percentage points or more.

Democrats also have a shot to pick up numerous seats in the Midwest. Democrat Abby Finkenauer is forecast to win Iowa's 1st District by greater than 5 points. Democrats also are favored to win in Kansas' 2nd and 3rd districts.

In the South, Democrats have a number of opportunities. They are within 2 points of longtime Republican incumbents in Texas' 7th and 32nd districts. They are are slightly ahead or tied in Kentucky's 6th District and Florida's 27th District.

Finally, in the West, Democrats are in good shape. In California alone, they are up (even if just barely) in the 10th District, 25th District, 39th District, 45th District and 49th District. Democrats probably won't win every single one of those (because most of them are still so close and some could go the other way), but it is possible.

There may be some surprises too. House races can break late, and the district polling from them isn't always reliable.

The Senate map is something altogether different. Unlike the House, where all 435 seats are up for re-election, only 35 of the 100 Senate seats are up for election this year. Democrats already hold 26 of them, which means they have limited ground to pick up the net two they need for control. Chances are they won't.

Democrats are favored by 1 point in Arizona, a seat currently held by retiring Republican Sen. Jeff Flake. Nevada bounces between the Democrat, Jacky Rosen, being slightly favored to Republican Sen. Dean Heller being forecast to win. Because Arizona and Nevada are close, it's far from certain that Democrats will win in either, let alone both.

A bigger problem is that even if Democrats do win both races they are forecast to lose in North Dakota. Democratic Sen. Heidi Heitkamp has trailed in every single poll taken over the last six months. Although the forecast has her down by only around 4 points (which is better than the double-digit deficit she has faced in some polling), she is a clear underdog.

A win in Arizona and Nevada but a loss in North Dakota would leave Democrats a seat short of a majority. To make up for a loss in North Dakota, they'd likely then need to pick off either Tennessee or Texas. That's possible. In those states, Democrats Beto O'Rourke and Phil Bredesen trail by 5 points or less. (Recent polling suggests our forecasts may be too Democratic.)

Here's the rub: Democrats have a bunch of seats where their candidates are up by less than 5 points. Beyond Arizona, Democratic Sens. Joe Donnelly of Indiana and Claire McCaskill of Missouri are up by 3 points or less. And Democratic Sen. Bill Nelson faces a tough challenge from Republican Gov. Rick Scott in Florida. Democrats may be expecting too much if they think their candidates will win in all of these races.

Still, winning all the close seats has happened before. Back in 1986, for example, Democrats won half a dozen Senate elections by less than 5 points.

The best chance Democrats have is if there is a shift in the national political environment. Such a change could give them a real shot at winning a lot of close races. It's not out of the realm of possibility that Democrats pick up a net gain of four Senate seats. It's just not likely.

On the other hand, if there is a change in the national environment against Democrats, Republican could pick up a net gain of six Senate seats.

Such a shift in the Republicans' direction would almost certainly benefit them on the House side as well. Because there are so many close races (about 20 within 2 points as of this hour), a small national swing could upend everything. Republicans could end up holding on to their House majority with plenty of seats to spare.

That's why the House, like the Senate, is still up for grabs.