Tim Kaine, George Allen debating in McLean

Democrat Tim Kaine and Republican George Allen face the first of several potential make-or-break moments Thursday in their pivotal U.S. Senate race in Virginia.

Kaine and Allen clash before northern Virginia's business elite at the Fairfax County Chamber of Commerce debate with a live regional television audience that includes the nation's capital and Virginia's most populous area.

The hourlong debate in McLean is going on now. Its moderator is David Gregory, host of NBC's "Meet The Press."

The debate comes one day after two polls showed Kaine appearing to open a lead over Allen.

One poll, conducted by the Post and published Wednesday, shows Kaine ahead of Allen by 8 percentage points, a finding that both campaigns reject. Another, by Quinnipiac University, showed Kaine with a 7 point lead.

Republicans dismissed the polls as unrealistically heavily weighted toward Democrats, and Kaine's campaign sent supporters an e-mail Wednesday night saying the race is much closer than the polls indicate and admonishing them not to become complacent with 48 days left until Election Day.

Thursday's debate is one of three in the stretch run of the campaign. Another is scheduled on Oct. 8 in Richmond and a final debate comes 10 days later on the Virginia Tech campus.

The Fairfax Chamber of Commerce Debate has become a command performance debate in each year featuring a statewide race.

The organization's members are among the most monied and politically muscular in Virginia.

It has served up defining moments and turning points three times in the past seven years, with some of them creating headlines beyond Virginia.

In 2005, Kaine benefited from Republican Jerry Kilgore's stumble when presented hypothetical questions by that year's moderator, the late "Meet The Press" host Tim Russert.

Kilgore had balked at Russert's question about whether he would sign legislation banning abortion in Virginia should the Supreme Court's 1973 ruling that legalized abortion be overturned, calling it hypothetical.

Because Kilgore had been portraying Kaine as a tax-happy liberal, Russert followed up by asking Kilgore whether he would veto a tax increase, and Kilgore quickly and eagerly said he would "in a heartbeat."

Russert busted Kilgore on the spot, noting he had answered one hypothetical yet not another. Kilgore stood red-faced as the crowd laughed at his expense.

One year later, Allen bristled during his Senate debate with Democrat Jim Webb when a panelist asked him about reports that his mother was of Jewish ancestry.

Allen questioned the relevance of the inquiry and said, "My mother's French-Italian with a little Spanish blood in her."

In fact, Allen would acknowledge after the election, his mother had tearfully told him weeks before the debate that her father, Felix Lumbroso, was Jewish and a resistance fighter in occupied Tunisia whom the Nazis had imprisoned during World War II when she was a girl.

Etty Allen, his mother, said she had kept it secret from her children, fearing similar recriminations against them as they grew up in the home of a famed coach who led the National Football League's Los Angeles Rams and Washington Redskins.

And in the 2009 gubernatorial debate between Republican Bob McDonnell and Democratic state Sen. R. Creigh Deeds, a post-debate question-and-answer session with reporters became damaging for Deeds when he said he would allow for new taxes to finance new roads and highway maintenance.

Kilgore, Allen and Deeds all lost their respective elections in each of those years.

The Associated Press contributed to this story.