Republican Sen. George Allen has a 16-point lead over Democratic challenger Jim Webb in the latest independent statewide poll, published Sunday, but a fifth of the electorate is still undecided. The election is closely watched nationally as an off-year referendum on the embattled Bush presidency because Allen, one of Bush's most reliable Senate allies, is preparing a 2008 presidential bid. Last year, Allen voted in support of the White House more than 95 percent of the time.Forty-eight percent backed Allen and 32 percent supported Webb in the Mason-Dixon Polling & Research Inc. survey of registered voters likely to vote in the Nov. 7 election. However, 20 percent of the 625 respondents surveyed statewide by telephone July 25-27 said they had not decided between Allen, a former governor seeking a second Senate term, and Webb, a former Republican who was President Reagan's Navy secretary. The poll's margin of error was plus or minus 4 percentage points.