In recent CNN poll on the Arkansas Senate race, Sen. Mark Pryor (D) was either winning by nine points or losing by two points to Rep. Tom Cotton (R). Same poll, two wildly different results. The first numbers, which had Pryor leading, were among a sample of those registered to vote. The second set was among only those who say they are "likely" to vote in seven weeks' time.

What explains such a big gap?  Simply put: Republicans are far more enthusiastic than Democrats to vote this fall and that gap will likely mean a major GOP turnout edge.

Let's break it down.

Among all registered voters, more tend to favor Democratic than Republican candidates for Congress. But for several reasons (including age, race and political interest), Republican-leaning Americans are more likely to vote. The size of that turnout advantage ranges from election to election, depending on how motivated each party's voters are to show up.

Conveniently, a quartet of national polls this month offer an early read on this year's turnout dynamics and allow for comparisons to recent elections. Each poll reported results for the "generic ballot," which asks which House candidate respondents currently support in their district, among both registered voters and likely voters.

All four polls show Republicans performing better among likely voters than among registered ones. The shift in the Democratic-Republican margin from registered to likely voters within each poll gives a sense of Republicans' current turnout advantage. Each poll shows Republicans do better among likely voters, with the swing in congressional support ranging from three points in the Fox News poll (Democrats -4 to -7) to an eight-point swing in the Pew Research Center survey (Dems +5 to -3). The Washington Post-ABC News and CNN-ORC polls fall in the middle of that spectrum.

The average likely voter swing toward Republicans is 5.5 percentage points across the four polls, slightly smaller than Republicans' advantage in 2010 (a 6.3-point swing toward Republicans) but clearly larger than in other recent midterms like 2006 (a 1-point swing) or 2002 (a 2.5-point swing).

The latest national polls largely jibe with polling earlier this year that found Republicans with a significant turnout advantage, albeit smaller than in 2010. Results for basic vote intention are a cruder but clearer way to see what's driving the selection of likely voters. In the latest Post-ABC poll, registered voters supporting Republican House candidates were eight points more apt to say they are "absolutely certain to vote" than those supporting Democratic candidates; in 2010, the Republican advantage was 13 percentage points.

The Pew Research survey found a slightly smaller difference from 2010 when asking whether voters are likely to vote on a scale of one to 10 (10 labeled "definitely"). Voters supporting Republicans were 12 points more apt to say they will "definitely vote" than those backing Democrats; in 2010 the margin was 14 points.

While the latest raft of polls is an indicator of Republicans' turnout advantage, it's not a done deal just yet. Pollsters may modify their likely voter models closer to the election, with some including additional turnout indicators. In addition, generic House polls have proven a useful indicator in the past but have also have incorrectly estimated the total vote in Democratic and Republican House races. In 2006, for instance, many national polls showed Democratic candidates leading by mid-to-high double digits over Republicans on the generic ballot, but when the votes were tallied, Democrats' edge shrunk to eight percentage points. It's possible that likely voter screens were too loose (meaning they let too many people in that weren't actually likely to vote) that cycle — always a big danger in low-turnout elections.

Still, the numbers are striking. And worrisome for Democrats.

Peyton M. Craighill contributed to this report.