The CBS Battleground Tracker has some bad news for Democrats. While they are likely to gain about 20 seats, that falls short of winning a House majority.

Twenty seats is right in the ballpark of historical losses for the party of a first term president in the off year election. The average number of seats lost by the majority party has been 23. But the Democrats need 24 seats to flip for them to take control of the House. While some prognosticators think that the Democrats have a House majority in the bag, have they really looked at the underlying data from 2016 - an election that virtually no "experts" got right?

Studying the exit polls in 2016 for CBS News coverage of the presidential election, Salvanto saw what was happening – Hillary Clinton was losing ground, and voters who hadn’t cast ballots were not likely to vote for her.  Salvanto delivered his verdict to the networks anchors and producers: “This is a contested race.”

The late deciders gave Trump the victory, Salvanto claims. CBS’s final 2016 poll gave him a big lead among voters who decided at the last minute.

Salvanto told the New York Post that comparing elections to horse races is wrong.  “The horse-race analogy is compelling but it’s wrong. “In a campaign, everything can change tomorrow.”

Based on that perspective, Salvanto is relying on the lessons of 2016 to watch this year’s midterms.  The CBS Battleground Tracker is looking at districts that will make a difference  He told the Post: “Remember, in a midterm, you have to watch each congressional seat — don’t pay attention to national numbers.”

The CBS News Battleground Tracker has a panel of 5,700 registered voters, with the majority in the districts that are highly contested. That poll indicates that few House seats will change parties in November, with the GOP likely holding its majority in the House.

“Right now I think this election looks like a toss-up,” Salvanto said to the Post. “We see a Democrat pickup in the House of Representatives in the 20-odd seat range, but Republicans could certainly hold on to the House.”

For there to be a "blue wave" in 2018, some of those districts rated as "solid" Republican would have to flip. It varies from pollster to pollster, but generally speaking, if a GOP incumbent received 55% of the vote or better in 2016, that seat is considered a likely or solid Republican hold.

The GOP's big problem and what may throw all predictions out the window, is the number of open seats in Republican districts that are being contested. Here, the margin of victory of the retiring incumbent doesn't mean as much.

Depending on how you count,¹ 39 Republicans and 18 Democrats are not running for re-election. That includes 13 Republicans and 10 Democrats who are leaving to seek another office, such as governor. Excluding them, 26 Republicans and eight Democrats are walking away from their political careers at the end of the 115th Congress. That’s the most “pure” retirements by Republicans — and the fewest by Democrats — since the 2008 election.

Democrats appear to have recruited many good candidates who are well financed. It seems certain that control of the House will hinge on how well the GOP does in holding on to seats in these open races.