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.
The Obama campaign dominated early voting here when it won the state in 2008. But Republicans say their side has caught up thanks to the high-turnout 2010 midterms and a same-sex marriage ban that was approved by voters earlier this year.
Watch for whether national parties pull their ad buys in this race if it looks hopeless, like Democrats did with Rep. Larry Kissell (D-N.C.) as of mid-October. It would be a devastating blow for either candidate before Oct. 18.
3. Ohio's 16th district Early voting starts Oct. 2
This is the most competitive House district in the most fought-over state in the presidential campaign. Every vote counts in the matchup between Reps. Jim Renacci (R) and Betty Sutton (D).
Outside groups descended on the district early, and Renacci has been on the air since late August. So it's curious that Sutton's camp has not started its own ad campaign, given that voters start heading to the polls Tuesday. Sutton's first ad buy is for Oct. 17, but Democrats say she will start earlier than that date.
The campaigns will be watching early vote totals in southern Cuyahoga County, which accounts for about half of the redrawn district's population. A strong early vote there could mean a victory for Sutton - and a poor turnout may signal Renacci's likely return to Congress.
4. Arizona Senate and 9th district Early voting starts Oct. 11
You'd be hard-pressed to find a state that makes early voting easier that Arizona. State officials automatically mail ballots for every election to voters who sign up for the Permanent Early Voting List.
Local operatives expect as many as 60 percent of voters to cast early ballots in the Senate race. That means the open-seat race between Rep. Jeff Flake (R) and former U.S. Surgeon General Richard Carmona (D) will be decided well before Election Day.
The most competitive House race in Arizona is the 9th district, which covers suburbs of Phoenix, including Tempe and south Scottsdale. It's a new district, so it's difficult to predict early voter turnout in the contest between Kyrsten Sinema (D) and Vernon Parker (R). But in all of Maricopa County in the 2008 general election, 55 percent of voters voted early.
5. Iowa's 3rd district Early voting started Sept. 27
Voters are already casting early ballots (mostly absentee) in this marquee Member-vs.-Member race between Reps. Tom Latham (R) and Leonard Boswell (D).
Twice as many 3rd district Democrats as Republicans have returned their ballots so far, according to preliminary figures from the Secretary of State. But Democrats have also requested more than three times as many ballots - 32,300 - as the GOP.
Boswell's team expects about a third of his supporters to vote early - the same as his 2010 tally. The Latham campaign declined to divulge their early voter targets, noting - correctly - that Republicans traditionally prefer to vote on Election Day in Iowa.
6. Colorado's 6th district Early voting starts Oct. 22
The state has a permanent mail-in voter list, and residents are taking full advantage of it. About 75 percent of voters opt to receive their ballots via mail, according to local operatives, who estimate an additional 10 percent will vote early in person.
The percentage of people signed up for the mail-in voter list is slightly lower in rural areas. So this suburban House battle between Rep. Mike Coffman (R) and state Rep. Joe Miklosi (D) is ground zero for the early vote operation.
7. North Dakota Senate Early voting starts Oct. 22
Residents prefer to vote early via absentee ballot in the Peace Garden State, and the lax voting laws make the Senate race between Rep. Rick Berg (R) and former state Attorney General Heidi Heitkamp (D) all the more interesting.
North Dakota has no voter registration: Voters only have to show proof of residency, such as a utility bill, at the polls. To vote absentee, voters can sign an affidavit with their ballot starting Sept. 27.
The early vote is particularly important in North Dakota because of the small population. Operatives expect about 350,000 ballots to be cast in the Senate race.
8. Any California House race Early voting starts Oct. 8
Take your pick of about dozen competitive races in the state: Rep. Dan Lungren's (R) campaign in the 7th district, the open-seat race in the 26th district, or Rep. Brian Bilbray's (R) re-election in the 52nd district, to name a few.
Early and absentee voting will play a role in all of these races - but it will be most important in places with large minority populations. For example, Rep. Mary Bono Mack (R) is in a competitive race in a district where a quarter of the voting age population is Hispanic.
9. Florida's 18th district Early voting starts Oct. 27
Early voting is important in every competitive Florida House race because of the state's large absentee ballot and minority populations.
But early ballots matter especially in the 18th district: Rep. Allen West (R) is currently a slight favorite over businessman Patrick Murphy (D). Absentee ballot operations are key because of the large "snowbird" population north of West Palm Beach. There's a sense of urgency among Republicans to have these older voters cast their ballots early, even though these part-time residents don't typically head south for the winter until after Nov. 6.
10. Illinois' 11th district Early voting starts Oct. 22
The Land of Lincoln has made significant changes to its voting laws in recent cycles. The legislature passed a law to start early voting in 2005, and last cycle the state dropped its requirement for voters to give a reason to vote absentee.
The state features a few competitive House races around the Chicago suburbs - typically prime territory for early voting turnout. But this race between Rep. Judy Biggert (R) and former Rep. Bill Foster (D) could be particularly affected by the state's early voting law. There's a growing minority population around Aurora that makes the 11th fertile early vote territory.
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.