(CNN) All the signs suggest that 2018 is going to be a very good year for Democrats.

Midterm elections are historically terrible for the president's party. In 18 of the last 20 midterm elections, the president's party has lost seats. In those 18 elections, the average seat loss is 33. Those numbers are even more daunting for presidents under 50% job approval -- as Donald Trump is right now. Since 1946, the average seat loss in the House in that situation is 36 seats

States up for re-election -- Blue: Democratic. Red: Republican. Yellow: Independent.

But before Democrats get too delirious about the election to come, they should read this paragraph from David Wasserman's terrific analysis of the 2018 election on FiveThirtyEight

"Even if Democrats were to win every single 2018 House and Senate race for seats representing places that Hillary Clinton won or that Trump won by less than 3 percentage points — a pretty good midterm by historical standards — they could still fall short of the House majority and lose five Senate seats."

That's absolutely stunning. And reflective of the advantages Republicans have going into 2018 -- one, in the House, built on having largely controlled the 2010 redistricting process, and the other, in the Senate, based on how great the 2006 and 2012 elections were for Democrats.

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