CNN's average of several recent national surveys indicates Obama, D-Ill., has a 3-percentage-point lead over U.S. Sen. John McCain, R-Ariz., 46 percent to 43 percent, which is half of his advantage in the "poll of polls" a week ago and down from 8 percentage points in July, CNN said.

"Over the last week, we've seen Senator Obama's lead in the poll of polls cut in half," said CNN Senior Political Researcher Alan Silverleib. "This change was likely driven by a renewed focus on foreign policy after Russia's invasion of Georgia, as well as by Senator McCain's willingness to launch more aggressive attacks against Obama on issues such as off-shore drilling."

Obama's slippage comes as both parties gear up for their national nominating conventions. The Democrats meet in Denver next week and the Republicans convene in Minnesota the week after that.