Polls show conflicting evidence for a Romney post-convention bounce

WASHINGTON - A week after Mitt Romney officially became the Republican nominee for president, opinion polls are split on the effect of the GOP convention.

This week Rasmussen Reports showed a 6-point bounce for the former Massachusetts governor, but Gallup produced a poll showing Romney's support unchanged. Why the disparity? There are several theories, but as New York Times statistician Nate Silver points out, the truth probably lies somewhere in between. Combining various poll results, Silver estimates Romney's actual post-convention bounce to be 2 or 3 percent.

Rasmussen currently shows Romney with 47 percent to President Barack Obama's 44 percent in national polling. Gallup has the president with a one-point advantage, but both results are within the margin of error.

Historically, presidential candidates see inflated poll numbers immediately after their party's nominating convention. The fact that Romney's bounce is harder to judge means the convention probably had less impact than Republicans were hoping.

Still, Romney can take solace in knowing Rasmussen has been one of the most accurate pollsters during the last several election cycles. Analysis of the 2008 presidential election shows Rasmussen and Pew Research Group were tied for the most accurate predictions, while Gallup ranked 20th out of 23 survey groups.

Virtually all current surveys reflect polling of the general population, not registered voters. Gallup calls this fact Romney's "secret weapon" because Republicans are showing more enthusiasm than Democrats for the first time since 2000. Once the polls reflect registered voters, Gallup expects Romney's numbers to improve.

Although the media often trumpet post-convention bounces, they are rarely an accurate indicator of election outcomes. John McCain out-bounced Obama (6 to 4 percent) after the conventions in 2008, and Walter Mondale more than doubled Ronald Reagan (9 to 4 percent) in 1984, only to lose by a significant margin.

George McGovern in 1972 and John Kerry in 2004 are the only other candidates to generate no post-convention bounce, according to Gallup.

The success of the Democratic National Convention likely hinges on Obama's speech tonight, but some Democratic operatives have already said they aren't expecting a big boost.