So a late polling shift toward the president’s party is not only unusual, when it has happened, it has tracked with unusually strong outcomes for the president’s party. However, shifts in momentum after August have happened before, in both directions. That means Democrats cannot presume that their positive summer trend will automatically continue into the fall.
Beyond the unknown of future news developments, Democrats don’t know how big a lead in the generic ballot poll average they need to keep the House. Conventional wisdom, as articulated recently by CNN polling editor Ariel Edwards-Levy, is that “Democrats typically need a healthy lead on that metric to translate into winning numbers in the House … partly because of redistricting, and partly because Democratic voters tend to live in more densely populated areas.”
It happens to be true that Democrats had big generic ballot poll leads in the final RCP average the last four times they won the House: 11.5 points in 2006, 9 points in 2008, 7.3 points in 2018 and 6.8 points in 2020. But in the first three of those four wins, the Democratic share of the House popular vote and, most importantly, House seats, was not narrow. Democrats didn’t need such big poll margins to reach the magic number of 218 House seats.
Only in 2020 did the Democrats win a majority of House seats with just a single-digit margin. And that year, the Democratic margin of victory in the House popular vote (3.1 %) trailed their final margin in the RCP average. That one example does suggest Democrats need a cushion in the polls. But it is not the case that the Democrats’ House popular vote performance always trails their poll performance. In the 10 House elections for which RCP produced a generic congressional ballot average, Democrats outperformed the poll average four times.
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