The Forecast: This is how difficult it is for Democrats to take the Senate

Senate forecast: Republicans will hold 52 seats (and maintain control of the Senate) next Congress while Democrats will hold just 48. Anything between Republicans holding 48 seats and 56 seats is within the margin of error.

Democratic hopes to take back the Senate remain dim. Perhaps, there's no way to understand that better than to look at the following Senate races: North Dakota, Tennessee, Texas and the Mississippi special election.

In the Democrats' best case scenario, they need to win every other close race and then one of these four. The problem is that the forecast looks bad for Democrats in all four of these.

A close examination of all of them reveals that in the closing days of the campaign, the races seem, if anything, to be shifting away from the Democrats.

North Dakota: Democratic Sen. Heidi Heitkamp hasn't led in a poll since February. Because of that, she's forecasted to lose by 7 points in our forecast. Trump won North Dakota by more than 35 points, and Heitkamp didn't help her cause with some late stumbles. Heitkamp has to hope that her large fundraising advantage indicates some underlying strength in her candidacy not picked up by the polling data. That seems unlikely at this time, and she looks destined to be a one-term senator.

Tennessee: Former Democratic Gov. Phil Bredesen is of a different era. He easily won re-election in 2006, and Democrats had hoped the goodwill left from his governorship would override the fact that Trump won the state by 26 points. It did, for a while. Bredesen led in most of the polling through September. Since that point, however, he's been trailing in most of them. The Forecast has him losing by around 5.

That margin, however, doesn't do justice to the spread in the data. Bredesen actually led in one recent public poll released of the race. Something that cannot be said for any of the other states listed above. Still, this poll is an outlier compared to most, including a Marist College poll released on Tuesday.

Texas: Democrat Beto O'Rourke, the hero of liberals in Austin and nationally alike, has never led Republican Sen. Ted Cruz in a high-quality poll of the race. O'Rourke's polling position may have even become worse of the last month, which is why The Forecast has him down by about 7 points. The only real reason to think O'Rourke stands much of a shot is how much money he has brought into campaign. In the end though, the state's natural partisan tilt (e.g. no Democrat has won statewide since 1994 and no Democrat has been elected senator since 1988) seems to have taken over.

Mississippi special: We probably won't actually know if Democrats' prayers here are dashed until late November. That's when a likely runoff between Republican Sen. Cindy Hyde-Smith and Democrat Mike Espy will take place for this seat formerly held by Republican Thad Cochran.

The runoff will be necessary because Cochran's resignation started a process in which a jungle primary (in which all candidates run against each regardless of party affiliation) takes place on Election Day. The top two-vote getters (if no one hits 50% in round one) advance to the runoff. Democratic chances rest upon another Republican (Chris McDaniel) reaching the runoff against Espy.

Polling indicates Hyde-Smith, however, as the most likely person to face Espy in the runoff. Even in a good political environment for Democrats nationally, it's nearly impossible for them to win a statewide federal contest in Mississippi unless the Republican is fatally flawed. Hyde-Smith isn't.

Of course, thinking that Democrats will need just one of these states may be a bit of magical thinking.

Contests such as Arizona, Florida, Indiana, Missouri and Nevada are within 3 points.

If Republicans win three out of these five, they'll end up with 53 seats in the next Congress. If Republicans sweep them, they'll be at 55 seats.

Our forecast thinks Republicans will win two of them and end up with 52 seats.

Even if Democrats win all five, Republicans would still be at 50 seats. With Vice President Mike Pence casting a tie-breaking vote, Democrats are still in the minority.