The poll -- based on interviews with 2,733 voters Friday, Saturday and Sunday -- found Obama preferred by 49 percent of those surveyed, compared to 43 percent for McCain, Gallup said on its Web site.

It was the first three-day poll for which all interviews were conducted after the Democratic National Convention in Denver. Going into the convention, Obama was preferred by 45 percent of registered voters, so he got a 4-point bounce out of the convention, Gallup said Monday.

This is just the second time Obama has reached 49 percent in the poll. He previously did it in late July.

The poll also reflects voter preference in the first three days following McCain's announcement of Alaska Gov. Sarah Palin as his running mate. At 43 percent, McCain is 2-points better than the 41 percent he polled before the running-mate announcement.

McCain's 2-point bounce is lower than the typical bounce for a vice presidential announcement, Gallup said, while noting that Obama got no bounce after announcing he had selected Delaware Sen. Joe Biden for the Democratic ticket.

The maximum margin of sampling error is plus or minus 2 percentage points.