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ABC News' Shushannah Walshe and Christopher Good report, with FiveThirtyEight.com's Harry Enten:

WHO'S ON THE BALLOT?

Voters in Georgia will cast ballots in primary run-offs today. One race, the GOP run-off for the open Georgia Senate seat, will set up a 2014 fight between tonight's victor and a Democrat with a familiar name in the Peach State. No incumbents face the chopping block tonight and while the Senate face off is the marquee race, it's not the only one to watch. Our friends at FiveThirtyEight.com have joined us again to explain the importance of today's big race. Look for FiveThirtyEight.com senior political writer Harry Enten's take below.

Here are three races to watch today:

GEORGIA'S GOP FEUD: Two months of nonstop Republican-on-Republican badmouthing will finally end in Georgia on Tuesday. With a merciful runoff vote, Georgia Republicans will choose either former Dollar General CEO David Perdue or Rep. Jack Kingston as their candidate for the state's open Senate seat to replace retiring Sen. Saxby Chambliss. WHY IT MATTERS: The top two finishers in a seven-way May 20 primary were Perdue, who received 30.6 percent of the vote, and Kingston, who came in second with 25.8 percent. Perdue has assailed Kingston's 21-year record of earmarks, while Kingston has pointed to layoffs, offshoring, and a government bailout among companies with which Perdue was involved, with fact checkers tweaking parts of his claims. Both candidates are vying, as one would expect, for the mantle of "true conservative." This isn't a traditional Tea Party vs. establishment GOP fight as we've seen in primaries all over the country, most notably the Mississippi Senate primary. Rep. Jack Kingston has collected a wide array of endorsements, from former primary foe Karen Handel-the Sarah Palin-endorsed tea partier in the multi-way first-round primary-to conservative blogger Erick Erickson, the NRA and even the Chamber of Commerce. Perdue is backed by former presidential candidate Herman Cain, who called him his "brother from another mother," because they're so similar politically. No Democrat has won a Georgia Senate seat since 2000, but the GOP infighting has left some breathing room for Michelle Nunn, the daughter of former Democratic Senator Sam Nunn, as Democrats have made good on their intentions to make the 2014 race competitive. After Republicans choose between two seemingly strong candidates, look for the attacks on Nunn to intensify and for a tough general election to get underway.

538's Take: Key Counties

Perdue will likely need to over-perform in the Atlanta metropolitan area. Perdue led Kingston by 17 to 18 points in Cobb (a traditional swing county in Republican primaries), Gwinnett, and Fulton counties in the first round. Handel came in first or second in all of these counties. If Perdue is to win the runoff, he'll need to fight off Handel's influence and win these counties by potentially upwards of 10 percentage points. He'll be building on his base anchored by Bibb and Houston counties around Macon in the middle of the state. Kingston, meanwhile, needs another strong performance around his Savannah (Chatham County) centered congressional district in the southeast. Kingston regularly won 75 percent or more of the vote and no less than 64 percent of the vote in 30 southeastern counties. More than that, turnout was up in these southeast counties, while it was down in most of the state. In Chatham, for example, turnout was up 7 percent from the competitive gubernatorial primary four years ago. It was down 11 percent statewide. Swing County: Augusta (Richmond County) in the center-eastern part of the state could be telling. It's just outside Kingston's sphere of influence in the southeast and Perdue's core support in the center of the state. Both candidates finished within a point of their statewide performance in Richmond in the first round.

THE OTHERS:

GEORGIA'S GOP RUN-OFF FOR THE 10 TH CONGRESSIONAL DISTRICT: Trucking company executive Mike Collins and pastor and conservative radio talk show host Jody Hice are facing off again today after the initial primary separated the two by less than one percent of the vote, with Hice leading by just a few hundred votes. Hice received 33.50 percent to Collins' 32.99 percent. WHY IT MATTERS: This seat is to replace the retiring Rep. Paul Broun, who ran unsuccessfully in the May primary for the U.S. Senate seat. Broun made a late endorsement of Hice, backing him just later this month. Collins is the son of former U.S. Rep. Mac Collins who represented Georgia in the House from 1993 to 2005. Mike Collins' father actually defeated Broun in the 1992 Republican primary congressional election, making it no surprise Broun backed Hice, although he said he waited because he did not want to anoint his successor. Collins has received the backing of former Speaker Newt Gingrich, former Pennsylvania Sen. Rick Santorum, and unsuccessful Georgia Senate candidate Karen Handel. The race hasn't been without its odd moment. A Vanilla Ice parody even popped up titled " Hice Hice Crazy," teasing Hice for some controversial comments he has made about women and gays. Collins has described himself as a successful businessman pushing that he can create jobs in the district, while Hice has been working the tea party vote, and cast himself as more of a cultural warrior. Eyebrow-raising comments Hice has made not only about women and gays, but also Muslims have been raised on the campaign trail. The winner will face Democrat Ken Dious, an attorney, but it's likely today's winner will also be the November victory in this bright red district.

GEORGIA'S GOP RUN-OFF FOR THE 11 TH CONGRESSIONAL DISTRICT: Former Congressman Bob Barr is trying to get back to Washington, facing off against former state Sen. Barry Loudermilk to replace the retiring Rep. Phil Gingrey, who also ran unsuccessfully for the Georgia Senate race. WHY IT MATTERS: Barr who served in the House from 1995 to 2003 is most well-known for helping lead the impeachment of President Bill Clinton and he's seeking a comeback in this seat north of Atlanta. Tea party-backed Loudermilk beat Barr, coming in the initial primary 6,000 votes ahead of Barr. Barr-who was the Libertarian candidate for president in 2008-was seen as the favorite thanks to his higher name recognition, but Loudermilk surprised with his initial victory and has even outraised Barr. Loudermilk went after Barr for writing a letter of recommendation six years ago for Attorney General Eric Holder. Barr says now he has called for his resignation. Barr has the backing of two of his primary opponents, while Loudermilk was backed by Sarah Palin. There is no Democratic candidate for the seat, so the winner of this primary is guaranteed to fill this district's congressional seat.

ABC's Caleb Jackson and Jennifer Hansler contributed to this report.

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