Control of the Senate comes down to just a few states, with Republicans in a position to pick up the necessary net six seats to win the majority.
As the results pour in Tuesday evening, here are the counties to watch in five of the most contested Senate races: Colorado, Iowa, North Carolina, Kansas, and Georgia.
Colorado: Arapahoe County and Jefferson County
These two Denver suburbs have served as bellwethers for statewide results in recent years, and probably will again as Sen. Mark Udall, a Democrat, fights to fend off Rep. Cory Gardner, a Republican.
Jefferson County’s results in the past six cycles have mirrored the statewide results within a percentage point. Arapahoe is another strong indicator of the statewide results in past years, but it’s also a county where there's often drop off in voters between presidential years and midterms. If the number of votes coming in from Arapahoe look similar to the vote total from 2012, it could be a good night for the Democrats. Political observers also say to watch Larimer County, home to Colorado State University, where campaigns have been making great efforts to turn out younger voters. Adams County is another one to watch, a Denver suburb that contains part of the 6th District, home to Colorado’s most competitive House race. Republicans say they don’t have to win there, but they can't afford to lose it by a lot. Kansas: Sedgwick County and Johnson County
It's been a while since Kansas last hosted a close Senate race, but that will change Tuesday when Republican Sen. Pat Roberts faces independent Greg Orman. The major battlegrounds are Sedgewick and Johnson Counties, home to more than a third of the voters in the Sunflower State. For Roberts to get re-elected, he needs to win both.
Sedgewick, where Wichita is located, has a Republican congressman — Rep. Mike Pompeo, R-Kan. — but the region's representation in the state legislature and senate is a mix of both parties. Orman is from Johnson County, but expect him to do well in traditional Democratic strongholds like Douglas, Shawnee, and Wyandotte Counties.
Roberts needs to run up the score in the 1st District, the state's largest that covers most of western Kansas. It’s a fount of reliable Republican votes, and the district Roberts represented before he jumped to the Senate. But Roberts has struggled with this conservative electorate in this run. Low turnout in the Big First, and tight races in Sedgewick and Johnson Counties, could spell trouble for the incumbent.
Georgia: Gwinett, Henry and Douglas Counties
Georgia’s rapidly changing demographics have helped put the state in play for Democrats. Those changes are on display in Gwinnett, a suburb northeast of the democratic stronghold of Atlanta. Gwinett used to be a predominately white Republican suburb, but its growing African-American population has given Democrats a foothold . In 2012, Mitt Romney won Gwinnett County with 54 percent of the vote.
Political observers say David Perdue, the GOP nominee, needs to win Gwinnett by that same percentage. If Democrat Michelle Nunn is taking 45 percent of the vote or more, she is still in the game .
Henry and Douglas are also traditionally Republican counties, where the increasing number of minority voters are changing the partisanship of the area. Romney won Henry County by 51 percent; Obama took Douglas County by the same margin.
North Carolina: Guilford, Wake and Mecklenburg Counties
Guilford is Sen. Kay Hagan’s home county, and she needs strong numbers there against Republican Thom Tillis. Hagan won there in 2008, but Sen. Richard Burr, R-N.C., took it in 2010. When Democrats lose statewide in North Carolina, they usually lose in Guilford. Hagan’s margins there will be an indicator of how the race is going .
However, Wake and Mecklenburg are important due to sheer size. Hagan will probably win both of them, but watch for Tillis’ margins. These are counties where Hagan can run up the score while Tillis scores big in the more rural parts of the state. Political observers say she needs to run up a lead of about a 15 percent or more in those counties. Tillis needs to stay above 43 or 44 percent to hold on.
Iowa: Polk County
As results come in from the Hawkeye State, political observers say they are watching regions of the state more than specific counties. But Polk County, the most populous in the state, and home to about 15 percent of the total vote — is on everyone's list of counties to watch. It's also home to the 3rd District , which features one of the country's hottest House races.
Rep. Bruce Braley, D-Iowa, will almost certainly win Polk County. But to mitigate Republican Joni Ernst's gains, Braley needs to win it by a wide margin. If he can run up the score, Ernst will have to put big numbers on the board in the rural counties, like Sioux and Woodbury, to win.
Correction, 12:21 p.m. A previous version of this story misspelled two counties in Kansas. They are Sedgwick and Wyandotte counties. Related: Republicans (Still) Poised to Pick Up 6 to 8 Senate Seats Billionaire Boys Club: Congress' Top Super PAC Donors Final Rankings: The 10 Most Vulnerable Senators Reid: Iowa Loss Would Mean Republican Senate Majority Early Voting Update in Four Senate Races Roll Call Election Map: Race Ratings for Every Seat Get breaking news alerts and more from Roll Call in your inbox or on your iPhone.