The Mason-Dixon Polling & Research Inc. found 46 percent of those interviewed supported Allen in his bid for a second term and 42 percent backed Webb. Twelve percent were undecided.
Because the margin of error for the poll is plus or minus 4 percentage points, the race could be seen as about even. Support for an independent candidate, Gail Parker, was not measured.
It was a dramatic narrowing of the 16-point lead Allen held in the same poll conducted in late July, when 48 percent backed Allen, 32 percent preferred Webb and 20 percent were undecided.
Allen, a cowboy-booted former governor and conservative backer of President Bush, has struggled with the president's low popularity all year. Allen's poll support has yet to crest 50 percent.
But Allen's real problems began Aug. 11 when, while addressing a rally of mostly white supporters in rural southwest Virginia, he twice referred to a Webb campaign aide, S.R. Sidarth, as "Macaca." The word denotes a genus of monkeys, including macaques, and is considered an ethnic slur in some countries.
Sidarth, a University of Virginia senior who was following Allen and videotaping his campaign events for Webb, recorded Allen's remarks. They were posted on the Internet and became a major national political story for more than a week.
Allen personally apologized to Sidarth, but the damage has been done, said Mark Rozell, a political science professor at George Mason University.
"The Macaca incident has made voters rethink their support for George Allen, and it's made it very hard for him to put out a positive message because he's been on the defensive for a month now," Rozell said.
In the poll conducted by phone last Tuesday through Thursday, Mason-Dixon interviewed 625 registered voters who said they are likely to vote in November.
Three percent said they would definitely not vote for Allen because of what he said. Eight percent said they were less likely to vote for him, 16 percent said they disapproved but would still support him and 28 percent said the comment was not so serious and they would vote for Allen. Ten percent were not sure and 37 percent said they wouldn't vote for Allen even if he hadn't made the gaffe.
Allen campaign manager Dick Wadhams said Allen has advantages polls can't measure: a tireless, one-on-one campaigning style and a well-organized statewide network of longtime volunteers.
Webb's campaign spokeswoman, Jessica Smith, said the poll proves Webb's claim that Allen's message is losing its grip on Virginia voters.