U.S. Democratic presidential candidate Barack Obama's lead over his Republican rival, John McCain, has declined by half, according to a poll released on Wednesday.

The CNN poll of polls showed that Obama only has a slim lead of 3 percentage points over McCain, at 46 to 43 percent, compared to a high of 8 points he had in mid of July.

CNN senior political researcher Alan Silverleib attributed the decline to "a renewed focus on foreign policy" after the crisis between Russia and Georgia, and McCain's "willingness to launch more aggressive attacks against Obama on issues such as off-shore drilling."

The poll combined three national surveys from Gallup, Quinnipiac and LA Times/Bloomberg.

However, the downturn is expected to cease since Obama will more media attention starting this week as he announces the running mate and officially get the party's presidential nominee.

"The big question now is whether Obama can successfully regain control of the campaign agenda as we head into the Democratic convention," Silverleib said.