Polls taken nationally and out of two swing states show a nail-biter. Here's our guide to today's polls and why they matter. 

Findings: While an NBC News/Wall Street Journal poll out today has the Obama ticket up 48 percent to the Romney ticket's 44 percent among registered voters, an Associated Press-GfK poll has Obama leading by one point — 47 percent to 46 percent — among that group.
Methodology: For NBC News/WSJ: Interviews with 1,000 registered voters August 16 through 20 with a margin of error of +/-3.1 percentage points. For AP-GfK: Telephone interviews with 885 registered voters August 16 through 20 with a margin of error with "95% confidence" is +/-4.1 percentage points. 
Why it matters: Just this month we saw national polls that gave Obama a cushy lead, but these two make the race appear much closer. Nate Cohn of The New Republic, writing on the NBC News/WSJ poll puts it as such: "The bottom line is that we head into the convention just about where we started: Obama up 4 among registered voters." Of the same poll Emily Schultheis at Politico said that showing the race at this distance is "is much closer to where most people think things stands right now." Josh Lederman and Tom Raum of The Associated Press write of their organization's poll focused on the fact that the addition of Paul Ryan to the ticket "has not altered the race against President Barack Obama. The campaign remains neck and neck with less than three months to go," adding: "The closely locked contest reflects deep partisan divisions across the country."
Caveat: Poll results within the margins of error. (See: Politico and Slate)


Findings: Obama leads by two points in Nevada — 47 percent to 45.
Pollster:  Las Vegas Review-Journal/KLAS-TV 8 NewsNow by SurveyUSA
Methodology: Automated telephone poll of 869 likely voters August 16 through 21 with a margin of sampling error of +/-3.4 percentage points.
Why it matters: As The Hill's Justin Sink points out, the poll sees Romney inching to Obama in the state — "That's the closest Romney has been to the president since an NBC News poll in May," he writes. Nevada is one of the 10 most important states in the Electoral College in the FiveThirtyEight forecast model
Caveat: Despite the closeness of the race, there's a divide as to how much Ryan helps the campaign: 28 percent said it made them more likely to pick Romney while 29 percent said it made them more likely to pick Obama.


Findings: Obama leads in Wisconsin 49 percent to Romney's 46 percent among likely voters. 
Pollster: Marquette Law School Poll
Methodology: Telephone poll of 576 likely voters August 16 through 19 with a margin of error of +/-3.8 percent. 
Why it matters: Just yesterday we wrote about a poll out of Wisconsin — Ryan's home base — that saw Romney leading by a point. The Marquette poll also gave Romney a lift in the numbers, but still shows Obama in a lead. Poll director Charles Franklin said: "The two-point shift in Romney’s direction is within the margin of error for the poll but suggests Ryan’s addition to the ticket may have slightly increased Romney’s chances in Wisconsin." In a piece published this morning Nate Silver concludes that he doesn't "think we should totally write off the idea that Mr. Romney has made up some ground in the swing states." 
Caveat: Among "all registered voters" the poll has Obama leading 50 percent to 44 percent.