Polling since 1964 suggests that both Mitt Romney and President Obama can expect small but significant increases in support following their respective party conventions. But sustaining those bumps is no sure thing.
Since 1964, the median “bump” or “bounce” earned by presidential candidates in Gallup polling after their conventions has been 5 percentage points. In some cases, the convention bump has receded quickly as presidential debates and subsequent events took over the story line. But in other years, a convention bump has set the tone for the final months of the campaign.
Four years ago, both Democrat Barack Obama and Republican John McCain received comparable increases in support following the party conventions. But McCain’s bounce subsided quickly with the advent of the crisis in the financial industry and his uneven response to it.
In some years, candidates have enjoyed no significant increase in support after conventions. For example, John Kerry in 2004 actually saw his support decrease by 1 percentage point in Gallup’s polling after the Democratic convention in Boston, while George W. Bush’s jumped only 2 points following the GOP confab in New York City.
But important and enduring bounces are also possible. Bill Clinton received an astronomical 16-point surge in support after Democrats gathered at Madison Square Garden in New York in mid-July 1992. In fact, Clinton captured the lead based on that bounce and never surrendered it. The 5-point jump in Republican George H.W. Bush’s support a month later after the Republican convention in Houston paled in comparison and was not enough to close the gap.
The magnitude of a candidate’s post-convention bounce is not predictive of the election’s outcome, however. McCain’s 6-point bump in support was slightly larger than Obama’s 4 points, but he was defeated in the general election. Democrat Walter Mondale out-bounced Republican Ronald Reagan by a 5-point margin in 1984 yet was defeated in a landslide.
Timing also matters. Comparing bumps in years when the nominating conventions were nearly a month apart with years when they occurred in successive weeks (both 2008 and 2012) is not terribly instructive. Four years ago, McCain tried to suppress Obama’s convention bump by announcing his choice of running mate, then-Alaska Gov. Sarah Palin, roughly 12 hours after Obama accepted his party’s nomination. At the outset of the Democratic convention, the two candidates were tied at 45 percent in the Gallup daily tracking poll. At the end of that convention, in Denver, Obama had moved out in front, 49 percent to 43 percent. But a week later, McCain had reversed that lead after the GOP convention in Minneapolis-St. Paul, passing Obama, 49 percent to 44 percent.
McCain, however, surrendered the lead immediately after financial giant Lehman Brothers filed for bankruptcy roughly a week after his acceptance speech. He never led again and lost in November by 7 points.
This year, pollsters face significant challenges in attempting to measure the effects of each party’s convention. The two conventions are separated by the Labor Day weekend, when many potential survey-takers are away from home and polling becomes difficult.
Additionally, polling after the Democratic convention will be complicated by the release of the August unemployment report. It will come out at 8:30 a.m. on Sept. 7, less than 12 hours after Obama accepts the nomination in Charlotte. That report, and the underlying economic news, could drive the two candidates’ poll numbers after their conventions—at least until the debates begin a month later.