Just a few months ago Democrats envisioned a “Blue Wave,” gaining 40-50 seats. Today their expectation have been tempered – and for good reason. Since 1946, Democrats have gained an average of 21 seats in the midterms when the president is a Republican. That gain would not be enough for a majority, where the Democrats need to gain 25 seats. Using election data since 1946, on average the president’s party (Republican or Democratic) loses 21 seats in the first midterm election. But that average masks big swings, from a 63-seat loss in 2010 to an 8-seat gain in 2002. The...