A new NBC/Wall Street Journal survey of voters shows President Obama with a sizable lead over Republican Mitt Romney, but that 6-point advantage comes with a very large asterisk.

In polling terms, it's called "over-sampling," and the Hart/McInturff team that a conducted the WSJ/NBC study over-sampled Democrats - by 11 percent.

In other words, substantially more Democrats than Republicans were polled, which could explain an advantage for Obama that exceeds anything recorded in other recent polls.

"Obama is not up by that much," GOP pollster Whit Ayres told The Washington Examiner.

Ayres posted a poll last week on his site, Resurgent Republic, showing Obama statistically tied with Romney, 47 percent to 45 percent. The poll showed the two men tied in 12 battleground states.

Ayres said over-sampling occurs randomly - so conspiracy theorists can put aside the notion that the poll sponsors were looking to give Obama a boost.

But 11 percent is about the highest over-sampling in recent memory, Ayres said. In the last 15 years, most exit poll over-sampling topped off at about 4 percentage points, with the exception of November 2008, when Democrats got a 7 percent advantage.

Ayres said the bottom line is most pollsters don't expect the voter breakdown on election day to hand Democrats an 11 percent advantage. It's more likely to be within a few points difference.

"Everything we are showing is that the Republicans are far more enthusiastic and energized about voting than Democrats," Ayres said.