Residents wait in line to pick up ballots at the courthouse in Waterloo, Iowa, when early voting in the state started on Sept. 27. Iowa's 3rd district race is one to watch: Democrats are voting early, but Republicans are likely to turn out on Nov. 6.
For many Senate and House races, the election could be over weeks before Nov. 6. If these Congressional candidates just aired their first commercials or haven't even debated their opponents yet, early voters might not hear any of it before making their decisions.
National campaign operatives anticipate 40 percent of ballots will be cast before Election Day this year. That percentage climbs even higher in states such as Arizona, North Carolina and Nevada, where early voting has become common practice.
"In Arizona, by and large, the elections are won beforehand in the early vote," said Rodd McLeod, a Democratic consultant in the Grand Canyon State.
Both presidential campaigns boast robust early voting operations. But the practice matters more further down the ballot, where the pool of potential voters is smaller.
So which party benefits from a strong early vote? It depends on the voting locations, hours and the "technical abilities of your local party culture," according to veteran GOP consultant Brad Todd.
"Republican do better when there are a lot of locations," he said. "Democrats do better when it's one location. They've always been a more structural turnout organization, and we've always been a more message turnout organization."
Early voting also benefits Democrats in dense urban populations with traditionally unreliable voting blocs and minority populations. To a lesser degree, it's popular among Republicans in the rural West, where it may be difficult to get to the polls on one specific day.
Here are Roll Call's top 10 places where early ballots will likely matter most in Congressional battles:
1. Nevada Senate and 3rd district Early voting starts Oct. 20
Democrats estimate two-thirds of the Silver State's votes will be cast early this cycle. Accordingly, the president's campaign is targeting the state's Latino population - which is rapidly growing but often an unreliable bloc at the polls - to cast early ballots. Rep. Shelley Berkley (D) hopes to reap the benefits in her competitive race against appointed Sen. Dean Heller (R).
Similarly, operatives expect between 50 percent to 60 percent of the population to vote early in the competitive 3rd district race between Rep. Joe Heck (R) and state Speaker John Oceguera (D). The district covers the southern portion of Clark County, where 60 percent of those who voted cast their ballots early in the 2008 elections.
2. North Carolina's 7th district Early voting starts Oct. 18
More than half of voters will cast their ballots early in Rep. Mike McIntyre's (D) very competitive re-election race. It's why outside groups have started investing in opponent state Sen. David Rouzer (R), including the YG Action Fund, which has put in $800,000.
Rep. Elijah Cummings, D-Md., right, hugs Harold Schaitberger, General President of the International Association of Fire Fighters, after the Congressman spoke at the IAFF's Legislative Conference General Session at the Hyatt Regency on Capitol Hill, March 9, 2015. The day featured addresses by members of Congress and Vice President Joe Biden.