Support for the President’s party on the generic House ballot is historically lower on election day than in the spring.
Most “generic ballot” polls attempting to forecast the 2018 general election have shown a narrowing of the gap between Democrats and Republicans in the past month or two. The gap was widest at the end of 2017 when Trump’s job-approval numbers reached their lowest point. While Republicans have regained a bit of ground since then, Democrats still have about a 5-7 point lead in recent polls.
Before Republicans get too excited by this rebound, we should examine the trends for the Trump Administration in the context of previous elections. Here are two charts that depict the trends in the difference between support for the President’s party and support for the opposition in generic ballot polls. The data for 2006 and 2008 comes from RealClearPolitics; the remainder comes from the archives at HuffPost Pollster that I have used in earlier postings. In each case I have averaged polls by month, combining together May and June, and July and August, when polling is less frequent than in the fall.
For both Obama years, 2010 and 2014, we see a fairly linear decline in the Democrats’ margin over the Republicans on the generic ballot. Unsurprisingly the fall in Democrats’ fortunes was much more substantial in 2010, when Obama observed that his party had been “shellacked” in the midterm.
In 2006 the Republicans faced a double-digit deficit in the spring. Though they shaved a couple of points off the Democrats’ lead by September, the Mark Foley scandal ended any hopes of a Republican come-back.
The Republicans begin the summer of 2018 facing a five-point deficit which might not be enough to swing the House of Representatives to the Democrats. If the history of past presidents is to be believed, though, the Republicans’ prospects may worsen as we head into November.