Early voting is playing a crucial role in several of the cycle’s most contested races for the Senate, where control hangs in the balance ahead of Tuesday’s midterm elections.  

Senate Republicans must gain six seats to win the majority — an increasingly likely scenario .  

Here’s a look at how both sides are faring in early voting in four of the most competitive Senate races:  

Colorado
Sen. Mark Udall, Democrat, vs. Rep. Cory Gardner, Republican.
Rothenberg Political Report/Roll Call Race Rating: Tilts Republican  For the first time, all state voters will cast their ballot by mail. That makes early voting in the Centennial State just, well, voting.  

As of Friday, Republicans undeniably have the advantage: 41 percent of early voters were registered Republicans, while 32 percent were registered Democrats, according to data from Colorado’s Secretary of State . But few operatives expect the margin to remain that wide.  

But Democrats attribute that advantage to geography and age. Older and rural voters are more likely to vote Republican — and more likely to mail their ballots early because it’s less convenient to physically turn in ballots on Election Day. Younger and urban voters are more likely to wait to turn in their ballots in person.  

Udall’s team say they remain optimistic, arguing unaffiliated voters — which the state’s data shows comprises 25 percent of early voters so far — will break for Democrats. Democrats also point to Republicans’ early lead in early voting 2010 and 2012, gaps that Democrats ultimately closed in statewide races on Election Day.  

“It’s a question of whether we can catch them this year fast enough,” said Colorado Democratic consultant Laura Chapin.  

Most of all, the new system, in which every single registered voter was mailed a ballot, makes turnout unpredictable. Voters can also register to vote on Election Day.  

Georgia
David Perdue, Republican, vs. Michelle Nunn, Democrat
Rating: Tossup Georgia does not have party registration, so political observers are analyzing Georgia early voting based on demographics. So far, results have been mixed.  

Democrats are pleased with the high number of African American voters turning out early — 30.4 percent of the ballots returned so far, according to the United States Election Project , which aggregates early voting data from every state. That’s a big deal for Democrats, according to pollster Fred Hicks, who said, “If the African-American turnout is 30 percent or higher, you can actually see Democrats win on the fourth.”  

But Hicks conducted an exit poll of early voters and found the majority of African Americans casting early ballots were already likely voters — meaning they were always expected to vote, they just opted to do so early. That’s a problem for Democrats, who put a lot of effort into turning out African-Americans in this midterm.  

The poll found Perdue leads in early voting, 50.6 percent to 44.8 percent for Michelle Nunn. Only 1.5 percent of respondents said they voted for libertarian Amanda Swafford, which is good for Perdue, who could lose support from the third-party candidate.  

Iowa
Rep. Bruce Braley, Democrat, vs. state Sen. Joni Ernst, Republican
Rating: Tossup As of Thursday, Democrats held a narrow lead over Republicans in early voting: 40.9 percent to 39.2 percent, also according to the United States Election Project .  

Iowa Republicans are encouraged by those results because historically, Democrats lead early voting by a much wider margin. Republicans credit their success to a concerted effort to prioritize early voting, arguing GOP voters are more motivated to turn out for Ernst than Democrats are for Bruce Braley.  

But Democrats say they remain confident. Iowa Democratic Party Communications Director Christine Freundlich said the GOP’s  “reliable voters who vote on Election Day are now just voting early” instead. Meanwhile, she argued Democrats are working to expand the electorate and get voters to the polls who would not normally turnout in a midterm elections.  

North Carolina
Sen. Kay Hagan, Democrat, vs. state Speaker Thom Tillis, Republican
Rating: Tossup The big question in North Carolina is how unaffiliated voters will break when the results are tallied. So far, unaffiliated voters are sending in ballots at a much higher rate than in 2010.  

Of course, both campaigns say their projections predict those voters will break for them — but that’s impossible to determine before Election Day.  

Registered Democrats are also voting early in higher numbers than they did in the last midterm — a good sign for Democrats.  

Tillis spokesman Daniel Keylin said those comparisons “don’t have much significance” because both 2010 and 2012 were terrible years for Tar Heel Democrats.  

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