Tim Kaine (Mark Gormus/Associated Press)

Allen (R) and Kaine (D) are their respective parties’ likely nominees in the contest to succeed retiring Sen. James Webb (D), and nearly every poll taken in the race has showed them in or near a statistical tie.

But the new NBC News/Marist Poll shows Kaine leading Allen, 48 percent to 39 percent, with 14 percent undecided. The same survey gives President Obama a 17-point lead over former Massachusetts governor Mitt Romney in a hypothetical November matchup, a result sharply at odds with other presidential polls in Virginia.

On Monday, the Allen campaign released a memo from its own pollster, John McLaughlin, arguing that Marist “structured the sample in a way that resulted in a strong partisan bias in favor of the Democrats. . . Historically, we have never seen an electorate that Democratic in Virginia in decades.”

The Marist survey came less than a week after the release of a Roanoke College poll that had Allen leading Kaine, 45 percent to 37 percent. Democrats — and, privately, many Republicans — said last week that they were skeptical of that poll as well, and strategists on both sides believe the race remains roughly even.