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.

Bush's job-approval rating is low even in Virginia, which last voted Democratic in a presidential election in 1964, the poll found. Forty-three percent rated Bush's performance as good or excellent while 56 percent judged it fair or poor. One percent of the respondents were undecided.

Bush's low popularity is a drag on Allen, conceded Allen's campaign manager, Dick Wadhams.

"It's a tough political environment all over. Virginia's no different in that it's a tough political environment for Republicans," Wadhams said.

Allen, 54, led Webb in every region of the state except its Washington, D.C., suburbs, Virginia's most populous, fastest growing and politically potent region. There, 43 percent supported Webb to 37 percent for Allen with 20 percent undecided, according to the poll.

"The bottom line is that Allen is consistently under 50 percent and that has got to hurt, being that he is a known entity in the state," said Webb spokeswoman Kristian Denny Todd.

Iraq was cited most often as the race's top issue. The continuing military entanglement was the first choice for 23 percent of the respondents, 19 percent cited the economy, 11 percent said it was the war on terror and homeland security, 8 percent cited education, 7 percent said government spending and taxation, and 7 percent said health care.

Webb, 60, a decorated Marine veteran of the Vietnam War and a best-selling author of war novels, has been a persistent critic of the war in Iraq. He left the Republican Party after the war began and campaigns extensively on the issue. He advocates a gradual U.S. withdrawal while Allen backs Bush's intent to keep troops there until Iraq's fledgling government can control insurgent attacks.

As of June 30, Allen's campaign had $6.6 million on hand compared to slightly more than $420,000 for Webb, according to Federal Election Commission reports.