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.
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.