Results from the Wall Street Journal-NBC News poll released Monday indicate the Illinois senator had an 8-percentage-point lead, down from 10 percentage points last week.
Fifty-one percent of likely voters said they favored Obama while 43 percent preferred the Arizona senator. Six percent said they were undecided, with about a third of this group indicating they were leaning toward a third-party candidate.
"The poll shows some slight movement for McCain. But with just 48 hours left, it's going to be a challenge to make up the rest of the difference," said Neil Newhouse, a Republican pollster who conducts the Wall Street Journal-NBC News survey with Democrat Peter Hart.
Thirty percent of voters already cast their ballot, the poll found.
The Wall Street Journal-NBC News poll was conducted Saturday and Sunday and has a margin of error of 3.1 percentage points.