Another thread exists, from a report expressing concern, about the 2018 Democrats and whether they win over a new majority for the U.S. House.
I predict they will.
It helps to understand, though, that it helps to keep one informed if one refers to the U.S. Popular Vote, for U.S. House, and that there is a target margin for the 2018 Democrats.
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2016 U.S. House: Republican 49.11% | Democratic 48.03% (Margin: Republican +1.08)
The 2016-to-2018 U.S. Popular Vote, for U.S. House, will shift in the direction of the Democrats.
In the midterm elections of 1946, 1954, 1994, 2006, and 2010—the last midterm election cycles in which the U.S. House flipped to the party opposite that of an incumbent U.S. President—saw an average of about +3.50 seats gained with each percentage point that was nationally shifted in the direction of the the minority party which won over a new majority. An example: In 2010, the Republicans, who flipped +63 seats, went into that midterm election with their 2008 loss of –10.60 (Republicans 42.62% | Democrats 53.22%), and then they won over the 2010 U.S. Popular Vote for U.S. House with a margin of +6.73. So, the 2008-to-2010 national shift, for U.S. House, was +17.33 percentage points in the direction of the Republicans. They won a net gain of 63 seats. So, 63, divided by 17.33, equaled +3.66 seats gained with each nationally shifted percentage point.
The 2018 Democrats, having lost the 2016 U.S. Popular Vote for U.S. House by –1.08, would win a net gain of +3 or +4 seats just by evening up the popular vote with a margin of +0.00 or winning with the minimum +0.01. The 2018 Democrats need a national shift in their direction of about +7 points, and to win the U.S. Popular Vote for U.S. House by a margin (whole number estimate) of +6 percentage points, in order to flip the U.S. House.
SCENARIO:
For those polls saying the 2018 Democrats will winning the U.S. Popular Vote for U.S. House by +7:
(+1.08) + 7 = 8.08 national shift | 3.50 x 8.08 = +28 seats gained
Commonly mentioned is a margin of +8:
(1.08) + 8 = 9.08 | 3.50 x 9.08 = +31 seats gained
For those polls which have reported as much as +14 in percentage points margin:
(+1.08) + 14 = 15.08 | 3.50 x 15.08 = +52 seats gained
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SUMMARY:
The 2018 Democrats need a net gain of +23 to win over the U.S. House.
My guess, should they prevail, is more in line with a 2016-to-2018 shift of about 10 points in order to win by +9.
(1.08) + 9 = 10.08 | 3.50 x 10.08 = +35 seats gained.