Polling since 1964 suggests that both Mitt Romney and President Obama can expect a small but significant increase 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 other outside factors dominate the storyline. But in other years, a convention bump has set the tone for the final few months of the campaign.
Four years ago, both Barack Obama and John McCain received comparable increases in support following their parties' conventions. But McCain's "bump" receded quickly amid the financial crisis and his uneven response to it.
This year, Romney can potentially receive two bumps — one following Saturday's revelation of his running mate, Rep. Paul Ryan, R-Wis., and another following the Republican National Convention set for Aug. 27-30 in Tampa, Fla. Polling released this week should reflect the extent to which the Ryan rollout boosted Romney's poll numbers, if at all. Even if the unveiling of his running mate provides the presumptive Republican nominee some momentum headed into Tampa, any increase in support may prove fleeting, as Obama and Democrats begin their convention just days later, on Sept. 3-6 in Charlotte, N.C.
Gallup editor-in-chief Frank Newport has studied how conventions affect the state of the race in his organization's polling. He calls the events of the next five weeks "a sequential three-bounce sequence."
"The meaningful data will be the contours of the race when the dust settles after all three bounce opportunities have occurred," Newport wrote in a blog post on Gallup's website.
Newport calls them "bounce opportunities" because, in some years, no significant increase in support occurred following a candidate's convention. For example, John Kerry actually saw his support decrease by 1 percentage point in Gallup's polling after the 2004 Democratic convention in Boston, while George W. Bush jumped only 2 points following the GOP convention in New York City.
But significant, enduring bounces are also possible. Bill Clinton received an astronomical 16-point increase 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 he never surrendered it. The 5-point jump in George H.W. Bush's support a month later after the Republican confab 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 who will win the election, however. McCain's 2008 bump (6 points) was slightly larger than Obama's (4 points), but he was defeated in the general election. Walter Mondale out-bounced Ronald Reagan by a 5-point margin in 1984, and he was defeated in a landslide.
Timing also matters. Comparing bumps in years when the nominating conventions were nearly a month apart with 2008 and 2012, when the conventions were in successive weeks, is not terribly instructive. Four years ago, McCain tried to smother Obama's convention bump by announcing his choice for running mate, then-Alaska Gov. Sarah Palin, roughly 12 hours after Obama accepted his party's nomination.
The two candidates in 2008 were tied at the outset of the Democratic convention in the Gallup daily tracking poll at 45 percent. At the end of the Democratic convention in Denver, Obama had moved out in front, 49 percent to 43 percent. But a week later, McCain had reversed that after the GOP convention in St. Paul, Minn., leading Obama, 49 percent to 44 percent.
McCain, however, surrendered his lead immediately after Lehman Brothers filed for bankruptcy roughly a week after his acceptance speech. He never led again and lost in November by a 7-point margin.
Looking ahead to this year's conventions, with polls differing at this juncture — Obama leads Romney by as many as 9 points in some polls, but the two are tied in the latest Gallup daily tracking poll — it may also be difficult to measure changes in each candidate's respective standing before and after the party gatherings. The two conventions are separated by the Labor Day holiday weekend, which is a difficult time to poll, as many potential respondents are away from home.
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. ET on Sept. 7, less than 12 hours after Obama accepts the nomination in Charlotte. That report, and the underlying economic narrative, could drive the two candidates' poll numbers after their conventions. At least until the debates begin a month later.