President Barack Obama is having more difficulty keeping 2008 supporters on his side than Mitt Romney is having holding onto John McCain voters, according to a new Gallup poll.

A survey of registered voters conducted July 23-29 found that 92 percent of Americans who voted for McCain in 2008 plan to vote for Romney this fall and just 5 percent have switched sides to support Obama. But among Obama's 2008 supporters, just 86 percent are planning to do so again and 9 percent are switching their support to Romney.

Overall 11 percent of voters who say they cast ballots for Obama or McCain are supporting the other party's candidate.

The margin of error for all 2008 voters is plus or minus 3 percentage points. For the sample of Obama or McCain voters, the margin of error is plus or minus 4 percentage points.