If it seems like John McCain and Barack Obama have been training serious fire on each other earlier than party nominees have in previous general elections, it's not just because passions are running high as a result of the Iraq war or the economy's woes. It also has something to do with the fact that the actual voting for President starts this week, and a record number of Americans are expected to pull the lever long before Nov. 4.

Even though bags of candy corn and other Halloween treats have barely hit drugstore shelves, Virginia voters will start casting their ballots on Friday at early-voting sites around the commonwealth. Another half-dozen states will open up early voting next week, before the candidates even meet for their first debate of the campaign. In all, 36 of the 50 states will allow early voting this year, including many key battleground states like Ohio and Colorado. As many as one-third of all voters are expected to make their selection before Election Day.

This new reality is upending traditional campaign strategies not just for the organizations of Obama and McCain but also for down-ballot candidates and ballot-initiative efforts. And it has hyper-compressed the presidential race. No sooner had the nominees selected their running mates and introduced them to the nation than they began pivoting to present their closing arguments, as Obama almost appears to be doing in his new 2-min. economy ad. In years past, candidates stayed on alert for an "October surprise" that could alter the race at the last minute. But in the brave new world of accelerated elections, any October surprise may come too late.

Presidential campaigns have long followed the same familiar calendar: a primary season, followed by party conventions, debates between the major-party candidates, a four-day intensive get-out-the-vote push that usually starts around Halloween and finally — after voters have sifted through ads and arguments or perhaps flipped a coin — Election Day. But in recent years the availability of early voting, whether by mail or in person (with some polling places open on weekends), has increased as voters have demanded a convenient alternative to waiting in long lines on the first Tuesday of November.

Because Election Day is not a national holiday, most Americans—with the exception of unionized workers like the Teamsters who have long negotiated to get the day off—have to rely on the indulgence of their employers or wait sometimes for hours in the evening in order to vote. A decade ago, only a handful of states offered early-voting options. Nationwide, 15% of the electorate cast early ballots in the 2000 presidential election. By 2004, 20% did so, and in the 2006 congressional elections, nearly one-quarter of all votes came from early voters.

At the state level, the percentages are even higher. Early voting accounted for almost 30% of Montana ballots in 2006 and 34% of votes in the state's primary this June. In Colorado, county clerks estimate that almost half of active voters in this year's general election will vote before Election Day; in some particularly competitive counties, as many as 60% are expected to vote at early-voting centers.

One of the puzzles campaigns are still trying to solve is who gets an edge from early voting. When the use of absentee ballots was largely limited to elderly shut-ins, Republicans were assumed to have an advantage. "Traditionally, Republicans have done a better job with mail-in ballots," says Mike Hamrick, chair of the Arapahoe County Democrats in Colorado. "We always used to say, Democrats win on Election Day and Republicans win in the post office." But as the popularity of early voting grows, the sheer number of voters involved makes that slice of the electorate more diverse. Polling in 2004 and 2006 has shown that those who utilize early voting still tend to be older, which would seem to help McCain—but they're also more educated and affluent, demographics that have supported Obama this year.

In addition, more early-voting centers are being located at colleges and universities, a change that significantly affects student turnout. Students at the University of North Carolina and N.C. State were able to vote on campus throughout the two weeks leading up to North Carolina's primary contest in April. At Duke University, however, students had to make their way to voting sites in the city of Durham. While turnout for Durham County was 52% in the Democratic primary, only 11% of eligible Duke students voted. This fall, however, Duke will have its own early-voting center, open for business starting Oct. 16.

Early voting can pose logistical challenges for campaigns. The quality of voter databases already varies widely among local party organizations, but now it's critical that candidates have a reliable way of eliminating the names of early voters from their target lists. (County clerk offices usually release those lists on a weekly basis.) "You don't want to chase people who have already voted," explains Nathan Chambers, chair of the Arapahoe County GOP. Early voting has also made exit polling in some areas decreasingly reliable as it fails to pick up large segments of the electorate.

The accelerated campaign season puts special pressure on down-ballot candidates. Voters traditionally gather information about local races and ballot initiatives late in a campaign cycle. With less time to define themselves and their opponents, local candidates may end up spending more money to cycle quickly through "meet the candidate" commercials and then contrast and attack ads before voting starts.

The shift can also affect how a campaign chooses to devote its resources and time on the trail. During the Democratic primaries, some of Hillary Clinton's senior advisers made a case for downplaying or even skipping Iowa, based on the argument that early voting would limit the importance of winning that state. "Iowans will not be the first to vote," Clinton's deputy campaign manager Mike Henry wrote in a memo.

And indeed, one reason for Clinton's comeback victory in New Hampshire was that her campaign had focused on nailing down early votes via mail-in ballots before Iowans even gathered at caucuses to make Obama their winner. Depending on your view, Oregon is either the ideal or the dangerous conclusion of the early-voting trend: in 1998 the state eliminated voting booths altogether and switched over to a 100% vote-by-mail system. Because ballots must be received, not just postmarked, by Election Day, most Oregonians make their decisions and send off their ballots long before those last few days that campaign professionals have traditionally used to persuade undecided voters.

Given the stakes in this election, many voters may still wait until the last moment rather than rushing to cast a ballot as soon as early voting begins in their state. After putting up with the race for this long, one would think, Why would voters head for the polls before even hearing the two presidential candidates address each other directly in a debate? Then again, after being subjected to almost two years of near endless campaigning and media coverage, the option of just voting and being done with it all may be harder to resist than either candidate's pitch.