Allen, positioning himself as a 2008 GOP presidential prospect even as he seeks a second Senate term, faces the former Republican and ex-Reagan administration Navy secretary before the Virginia Bar Association's summer retreat at the Homestead, an exclusive Allegheny Mountain resort.
The midsummer debate, held before a private audience on a Saturday morning without live television or radio, rarely generates broad public notice. Coverage is mostly relegated to daily newspapers.
But the burden is on Webb to make it memorable, at least to political insiders who track such things and fund campaigns, said Larry J. Sabato, director of the University of Virginia Center for Politics.
"For Webb, it's an important test to people who watch closely to see if he can stay toe-to-toe with the well-funded incumbent," Sabato said.
Allen finished June with $6.6 million in his campaign treasury, compared to just over $424,000 for Webb. That 15-to-1 cash advantage would allow Allen to bury Webb in television advertising this fall unless Webb, who has raised money during most of July, can give donors a reason to narrow Allen's monetary dominance.
"It's also important that he (Webb) do well because he only has a few opportunities to face Allen in person and to be an equal on stage with him," Sabato said.
The two campaigns have tentative agreements to meet only twice more: Sept. 17 in a one-on-one forum on NBC's "Meet The Press" with host Tim Russert, and the next day at the annual Fairfax County Chamber of Commerce debate.
Both candidates have spent much of the past week preparing for the debate, mostly with longtime friends and close political advisers coaching them, say aides to both candidates.
Frank B. Atkinson, a cabinet member when Allen was governor and a senior Justice Department deputy during the Reagan administration, and longtime political adviser Christopher J. LaCivita prepped Allen.
Expect Allen to continue portraying Webb as a man of shifting positions, particularly on the race's seminal issue: Iraq.
"He (Webb) has been a candidate for, what, six or seven months now, and content has been sorely lacking in his campaign. What positions he has taken have many times been contradictory," said Allen spokesman Dick Wadhams.
Webb wrote in a 2002 newspaper column that an Iraq invasion would create a deadly, long-term military entanglement for the United States. Webb left the GOP after the war began in 2003.
He has said U.S. troops could be withdrawn within two years but does not propose a deadline. Allen supports President Bush's intent to keep American forces in Iraq until the fledgling republic can quell the nation's bloody insurgencies on its own.
Webb, a decorated Marine veteran of the Vietnam War, is coached by longtime friend and former Marine Nelson Jones as well as Steve Jarding, his chief campaign strategist, and Mo Elleithee, a Democratic communications consultant who had advised Webb's primary opponent, Harris Miller. Jarding and Elleithee played key roles in Mark R. Warner's election as governor in 2001.
"We mostly kind of kept it in the family," said Webb spokeswoman Kristian Denny Todd.