On Monday Gallup released figures showing Republicans with their largest lead ever in the generic ballot. Fully 51 percent of registered voters said they preferred their Republican House candidates, as opposed to 41 percent who said they would vote Democratic. But the question remains: how predictive are these numbers for the November elections?
Historically, midterm elections commonly go against the presidential party. Since 1948, all midterm elections except for two--those in 1998 and 2002--have resulted in a loss of House seats for the president's party. Since the 1948 election, in presidential election years, the party that has won the presidency has on average gained 12 House seats. In midterm years, the presidential party has on average lost 22 House seats. This year, Republicans need to win 39 seats to gain a House majority--a steep but attainable goal. Poll results like Gallup's only bolster conservatives' confidence that they can flip at least that many seats.
There is good reason for the GOP to be optimistic at these new numbers. One is the numbers' unprecedented nature: prior to this year, the highest midterm Republican generic ballot leads had been five points wide, in both June 2002 and July 1994.
Additionally, as Gallup Editor in Chief Frank Newport said this week, "Gallup's generic ballot has historically proven an excellent predictor of the national vote for Congress, and the national vote in turn is an excellent predictor of House seats won and lost." Mark Blumenthal, Senior Polling Editor of The Huffington Post, agrees that the generic ballot is "a reasonably good guide to the outcome at the end [of an election]." In 1974, for example, when Republicans trailed by 20 points in generic ballot polling, they lost 43 House seats. In 1994, when Republicans achieved their historic five-point lead in generic polls, Democrats went on to lose 53 seats.
Of course, poll results must be taken in context. In week-to-week polling, results naturally fluctuate. As recently as mid-July, Democrats led with 49 percent in Gallup's generic ballot to Republicans' 44 percent. Furthermore, Blumenthal asserts, "I think this week's Gallup number is on the high end of the random variation" of generic ballot polling. That is to say, the wide Republican lead measured by Gallup is high, but also does not appear to be out of the bounds of the normal range of results seen from poll to poll. Similar surveys from other sources show how much results can vary. On August 30, generic House polling from Rasmussen Reports gave Republicans a six-point lead, with 45 percent of likely voters. An August 25-26 Newsweek poll of registered voters showed the two parties tied at 45 percent.
However, Blumenthal adds that when one accounts for all of this variation, "[t]he trend that you see underneath is that the generic ballot measure has been worsening for the Democrats over the course of the summer, and that's true regardless of the type of poll." Poll analysis website Pollster.com, which has averaged out the results from many different polls, shows a steady and widening Republican lead since May. Scott Rasmussen, president of Rasmussen Reports, believes this momentum will be difficult to stop. "Barring a major change in economic news, or some kind of foreign policy event, it's hard to see something that would dramatically shift these numbers," he says.
A poll of registered voters like Gallup's may even understate the magnitude of Republicans' advantage come Election Day. Polls of likely voters are generally considered more accurate because they also reflect each side's enthusiasm level. "This year, Republicans are more likely to show up and vote," Rasmussen says, noting that recent polls consistently indicate higher enthusiasm among Republican voters than Democratic voters. Blumenthal also says that the current polling landscape points to an unusually large turnover in the House. "Even if the generic-ballot gap between Republicans and Democrats is five or six points wide," he says, a House turnover to rival 1994 is "quite plausible."