My colleague David Wasserman has been digging into the question of just how big of a wave Democrats need to get in order to surf into the majority. The short answer: they need to see a generic ballot advantage of +8 or more, which roughly translates to getting at least 54 percent or more of the national House vote in 2018.
The last time Democrats enjoyed a margin of +8 or more in a mid-term year was 2006. That year, Democrats won the House vote by 8.5 percent. The last time that Democrats got into the double digits was 2008 when they carried the House vote by D+11. This has led to a lots of talk that Democrats can only hit significant margins of victory in presidential elections when their base is more engaged and involved. It also helped to have a transformational candidate - Barack Obama - at the top of the ticket. Something they obviously don't have in 2018. But, there is precedent for Democrats winning the House vote by double digits in mid-term years. In the post-Watergate midterm of 1974, Democrats won by a whopping 17 points. In Ronald Reagan’s first midterm of 1982, Democrats won the House vote by 12 points.Cook notes other evidence pointing to a wave:
The off-year election results also point to a wave. It was a foregone conclusion that Democrats would pick up the governorship of New Jersey, but state legislative gains in Virginia, Georgia, Washington, and elsewhere are ominous for Republicans. Tim Storey, the elections guru at the National Conference of State Legislatures, says that about 33 state legislative seats have shifted from Republican to Democrat this year, while just two have gone from Democrat to Republican, and a couple of seats could go either way. In special congressional elections, in strongly GOP districts in Georgia, Kansas, Montana, and South Carolina, Republicans have come out on top, but voting patterns showed them underperforming the norm by 6 to 12 points. These results do not bode well for the 23 GOP-held districts won last year by Hillary Clinton or for those that Trump won narrowly.