One thing presidential candidates hope they'll get from their nominating conventions is a healthy “bounce” – a gain in popularity as measured by public opinion polls. Since both parties' 1964 gatherings, candidates have enjoyed an average 10 percentage point gain in their margin against their opponent, based on calculations from figures provided by The Gallup Poll.

But this year could be different with the two conventions almost back-to-back.

Presidential hopefuls are usually eager to dampen the bounce their rival will enjoy. It's no coincidence that Republican John McCain is expected to announce his vice presidential running mate as early as Friday, a day after Barack Obama delivers his acceptance speech and the Democratic National Convention ends.

The post-convention boost, however, doesn't always last and is hardly predictive of election outcomes.

Since 1964, there's been no real difference between the bounce enjoyed by the two parties. Democrats have averaged an 11-point gain, Republicans 9 points.

There's also no significant difference in bounces by the party that doesn't hold the White House – which by tradition holds the year's first convention – and the incumbent party. The out-of-power party averages an 11-point increase, compared to a 9-point boost for the incumbent party.

The biggest boost was the 30-point increase Bill Clinton gained when running for president for the first time in 1992. He never relinquished his lead.

The largest for Republicans: the 14-point increases George W. Bush gained in 2000 and that Richard Nixon enjoyed in 1968, and the 13-point bounce Ronald Reagan got in 1980. All three won their first races for president.

The smallest bounces were the 4-point drop John Kerry saw in his margin against President Bush in 2004, and the 3-point reduction George McGovern endured in 1972 against President Nixon. Both Democrats lost.

These quick popularity boosts don't always mean much. In 1964, President Johnson and Republican Barry Goldwater saw convention bounces of about the same size, while in 1984 Democrat Walter Mondale's 16-point boost was double President Reagan's. Johnson and Reagan won landslides.

In 2004 Bush got the smallest GOP bounce measured yet – 2 points – and was re-elected.

Obama's bounce this year could be limited not only by McCain's expected selection of a running mate, but by the Republican convention itself, which begins four days after the Democrats' ends.

But McCain's bounce could also be stifled. Gustav, a storm in the Caribbean, was nearing hurricane strength Thursday and could hit the Gulf Coast early next week, potentially a major distraction.

And McCain's acceptance speech next Thursday night will have to compete with the nationally televised opening game of the NFL season, featuring the Super Bowl champion New York Giants.