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Both campaigns largely have shrugged off polls that appeared to give Mr. Kaine a slight edge in recent weeks, insisting that the race would remain within the margin of error to Election Day. Indeed, the latest Real Clear Politics average of Virginia polls gives Mr. Kaine a 2.2-point lead, at 47.6 percent to 45.4 percent — less than most polls’ error margins and with both men still well below the all-important 50 percent mark.

The contest between Mr. Kaine and Mr. Allen has seen more spending from outside groups than any other U.S. Senate race in the country — more than $22 million, according to the Center for Responsive Politics. With Democrats holding an effective 53-47 majority in the Senate, the race is one of a handful that could well determine which party controls the body come January.

When Mr. Allen officially entered the race nearly two years ago, he thought he was going to have a rematch with Democratic Sen. Jim Webb, who narrowly defeated him in 2006. But when Mr. Webb opted not to run for a second term, Mr. Kaine stepped in after Democrats across the state pushed the former governor to run.